diff --git a/README.md b/README.md
index 9101fed..1fd3607 100644
--- a/README.md
+++ b/README.md
@@ -14,6 +14,16 @@ just new my-slug     # scaffold a new post
 
 Needs Gleam 1.17+ and Erlang/OTP 28+ — `nix develop` provides everything.
 
+### Environment variables
+
+| Variable   | Default                | Description                          |
+| ---------- | ---------------------- | ------------------------------------ |
+| `BLOG_URL` | `https://karitham.dev` | Base URL for OG tags, RSS, and links |
+
+```sh
+BLOG_URL="http://localhost:8000" just build
+```
+
 ## Adding a post
 
 ```sh
@@ -28,6 +38,7 @@ Or create a directory manually:
 ```
 priv/posts/my-post/
   index.md
+  hero.png          # optional hero/banner image
   diagram.png       # → /posts/my-post/diagram.png
 ```
 
@@ -40,6 +51,7 @@ description: short summary
 date: 2026-07-18
 tags: [gleam, atproto]
 draft: true
+image: hero.png
 ---
 ```
 
diff --git a/gleam.toml b/gleam.toml
index 35bb522..c6b5f99 100644
--- a/gleam.toml
+++ b/gleam.toml
@@ -14,6 +14,7 @@ argv = ">= 1.0.0 and < 2.0.0"
 shared = { path = "shared" }
 atproto_client = ">= 0.1.0 and < 1.0.0"
 gleam_time = ">= 1.0.0 and < 2.0.0"
+yamleam = ">= 1.0.0 and < 2.0.0"
 
 [dev_dependencies]
 gleeunit = ">= 1.0.0 and < 2.0.0"
diff --git a/manifest.toml b/manifest.toml
index a1b2e08..da42bc7 100644
--- a/manifest.toml
+++ b/manifest.toml
@@ -174,6 +174,11 @@ packages = [
   ], requirements = [
     "gleam_stdlib",
   ], otp_app = "splitter", source = "hex", outer_checksum = "3DFD6B6C49E61EDAF6F7B27A42054A17CFF6CA2135FF553D0CB61C234D281DD0" },
+  { name = "yamleam", version = "1.0.0", build_tools = [
+    "gleam",
+  ], requirements = [
+    "gleam_stdlib",
+  ], otp_app = "yamleam", source = "hex", outer_checksum = "16213706D44FF1A0010ECD9E86F95E7F50E5C4BE4D61219512CF6A118E8CEC83" },
 ]
 
 [requirements]
@@ -191,3 +196,4 @@ lustre = { version = ">= 4.0.0 and < 6.0.0" }
 mork = { version = ">= 1.12.1 and < 2.0.0" }
 shared = { path = "shared" }
 simplifile = { version = ">= 2.0.0 and < 3.0.0" }
+yamleam = { version = ">= 1.0.0 and < 2.0.0" }
diff --git a/priv/static/style.css b/priv/static/style.css
index 73e0008..ba67b04 100644
--- a/priv/static/style.css
+++ b/priv/static/style.css
@@ -34,7 +34,8 @@
 
   /* surfaces — layered shadows for depth */
   --shadow-ring: 0 0 0 1px color-mix(in srgb, var(--text) 8%, transparent);
-  --shadow-depth: 0 2px 4px 0px color-mix(in srgb, var(--bg-nav) 50%, transparent);
+  --shadow-depth: 0 2px 4px 0px
+    color-mix(in srgb, var(--bg-nav) 50%, transparent);
 }
 
 /* ============================================================
@@ -275,6 +276,24 @@ footer {
   outline: none;
 }
 
+/* ============================================================
+   Post hero image
+   ============================================================ */
+
+.hero-image {
+  margin-bottom: 1.75rem;
+  border-radius: 4px;
+  overflow: hidden;
+}
+
+.hero-image img {
+  display: block;
+  width: 100%;
+  height: 280px;
+  object-fit: cover;
+  outline: none;
+}
+
 /* ============================================================
    Profile
    ============================================================ */
diff --git a/shared/src/api.gleam b/shared/src/api.gleam
index 939f3dd..956e2e5 100644
--- a/shared/src/api.gleam
+++ b/shared/src/api.gleam
@@ -1,5 +1,18 @@
 import gen/repo.{type Repo}
 import gleam/list
+import gleam/string
+
+/// Returns the site URL, preferring the BLOG_URL environment variable
+/// if set. Falls back to the hardcoded default.
+pub fn site_url() -> String {
+  case os_getenv("BLOG_URL") {
+    Ok(url) -> string.trim(url)
+    Error(_) -> "https://karitham.dev"
+  }
+}
+
+@external(erlang, "api_ffi", "getenv")
+fn os_getenv(name: String) -> Result(String, Nil)
 
 pub const pds_endpoint = "https://eurosky.social"
 
diff --git a/shared/src/api_ffi.erl b/shared/src/api_ffi.erl
new file mode 100644
index 0000000..479b31d
--- /dev/null
+++ b/shared/src/api_ffi.erl
@@ -0,0 +1,8 @@
+-module(api_ffi).
+-export([getenv/1]).
+
+getenv(Name) ->
+  case os:getenv(binary_to_list(Name)) of
+    false -> {error, nil};
+    Value -> {ok, list_to_binary(Value)}
+  end.
diff --git a/src/blog_tools.gleam b/src/blog_tools.gleam
index 828fa0f..4528d46 100644
--- a/src/blog_tools.gleam
+++ b/src/blog_tools.gleam
@@ -167,6 +167,7 @@ fn template(slug: String) -> String {
   <> "\n"
   <> "tags: []\n"
   <> "draft: true\n"
+  <> "image: \n"
   <> "---\n"
   <> "\n"
   <> "Write your post here.\n"
diff --git a/src/build.gleam b/src/build.gleam
index cf66508..895b092 100644
--- a/src/build.gleam
+++ b/src/build.gleam
@@ -1,11 +1,14 @@
+import api
 import data/fetch
 import data/model.{type Post, type SiteData, SiteData}
 import dynamic
 import encode
+import filepath
 import gen/actor/defs.{type ProfileViewDetailed}
 import gleam/io
 import gleam/list
 import gleam/option.{type Option, None, Some}
+import gleam/result
 import gleam/string
 import hydration.{HydrationModel}
 import lustre/attribute.{class, id}
@@ -92,7 +95,7 @@ fn write_index(data: SiteData) {
     layout.Meta(
       description: description,
       image: og_image,
-      url: "https://karitham.dev/",
+      url: api.site_url() <> "/",
       logo: data.profile.avatar,
       page_type: layout.Website,
     )
@@ -114,14 +117,14 @@ fn write_single_post(post: Post, profile: ProfileViewDetailed) {
   let title = post.title <> " - Kar"
   let og_image: Option(String) = case post.image {
     "" -> profile.avatar
-    img -> Some(img)
+    img -> Some(resolve_og_image_url(post.slug, img))
   }
 
   let meta =
     layout.Meta(
       description: post.description,
       image: og_image,
-      url: "https://karitham.dev/posts/" <> post.slug <> "/",
+      url: api.site_url() <> "/posts/" <> post.slug <> "/",
       logo: profile.avatar,
       page_type: layout.Article(published_time: post.date, tags: post.tags),
     )
@@ -237,6 +240,23 @@ fn copy_client_js() {
   }
 }
 
+fn resolve_og_image_url(slug: String, img: String) -> String {
+  case
+    string.starts_with(img, "http://") || string.starts_with(img, "https://")
+  {
+    True -> img
+    False -> {
+      let expanded = filepath.expand(img) |> result.unwrap(img)
+      case filepath.is_absolute(expanded) {
+        True -> api.site_url() <> expanded
+        False ->
+          api.site_url()
+          <> filepath.join(filepath.join("/posts", slug), expanded)
+      }
+    }
+  }
+}
+
 fn copy_dir(src: String, dst: String) -> Nil {
   case simplifile.read_directory(src) {
     Ok(entries) ->
diff --git a/src/data/fetch.gleam b/src/data/fetch.gleam
index 6a31252..8d9a2a1 100644
--- a/src/data/fetch.gleam
+++ b/src/data/fetch.gleam
@@ -180,6 +180,8 @@ fn format_parse_error(slug: String, e: frontmatter.ParseError) -> String {
       <> "\" has invalid date \""
       <> value
       <> "\" (expected YYYY-MM-DD)"
+    frontmatter.InvalidYaml(_, error) ->
+      "post \"" <> slug <> "\" has invalid frontmatter: " <> error
   }
 }
 
diff --git a/src/data/frontmatter.gleam b/src/data/frontmatter.gleam
index 23745c6..589a721 100644
--- a/src/data/frontmatter.gleam
+++ b/src/data/frontmatter.gleam
@@ -1,10 +1,12 @@
 //// Frontmatter parser for blog posts.
 ////
-//// Posts in `priv/posts/<slug>/index.md` start with a YAML-ish block:
+//// Posts in `priv/posts/<slug>/index.md` start with a YAML block:
 ////
 ////     ---
 ////     title: My Post
-////     description: short summary
+////     description: >-
+////       A short summary
+////       that can span lines.
 ////     date: 2026-07-18
 ////     tags: [gleam, atproto]
 ////     draft: true
@@ -14,22 +16,24 @@
 //// `ParseError` if anything is wrong). Pure — does no I/O.
 
 import data/model.{type Post, Post}
-import data/util
+import gleam/dynamic/decode
 import gleam/list
 import gleam/result
 import gleam/string
 import gleam/time/timestamp
 import mork
+import yamleam
+import yamleam/error
 
 pub type ParseError {
   MissingField(slug: String, field: String)
   InvalidDate(slug: String, value: String)
+  InvalidYaml(slug: String, error: String)
 }
 
 /// Parse raw markdown content (frontmatter + body) into a Post.
-/// Returns an error if a required field is missing or the date is
-/// malformed — the build fails loudly on those rather than
-/// silently producing a broken post.
+/// Returns an error if a required field is missing, the date is
+/// malformed, or the YAML is invalid.
 pub fn parse(slug: String, content: String) -> Result(Post, ParseError) {
   case string.split(content, on: "\n---\n") {
     [frontmatter, body] -> parse_with_frontmatter(slug, frontmatter, body)
@@ -42,84 +46,40 @@ fn parse_with_frontmatter(
   frontmatter: String,
   body: String,
 ) -> Result(Post, ParseError) {
-  let lines = string.split(frontmatter, "\n")
-  let title = util.extract_field(lines, "title:", "")
-  let description = util.extract_field(lines, "description:", "")
-  let date = util.extract_field(lines, "date:", "")
-  let tags_str = util.extract_field(lines, "tags:", "[]")
-  let draft_str = util.extract_field(lines, "draft:", "false")
-  let image = util.extract_field(lines, "image:", "")
-
-  case title == "", date == "", parse_article_date(date) {
-    True, _, _ -> Error(MissingField(slug, "title"))
-    _, True, _ -> Error(MissingField(slug, "date"))
-    _, _, Error(_) -> Error(InvalidDate(slug, date))
-    False, False, Ok(_) ->
-      Ok(build_post(
-        slug,
-        title,
-        description,
-        date,
-        tags_str,
-        draft_str,
-        body,
-        image,
-      ))
+  let decoder = {
+    use title <- decode.field("title", decode.string)
+    use description <- decode.optional_field("description", "", decode.string)
+    use date <- decode.field("date", decode.string)
+    use tags <- decode.optional_field("tags", [], decode.list(decode.string))
+    use draft <- decode.optional_field("draft", False, decode.bool)
+    use image <- decode.optional_field("image", "", decode.string)
+    decode.success(Post(
+      title:,
+      description:,
+      slug:,
+      date:,
+      content: "",
+      tags:,
+      draft:,
+      image:,
+    ))
   }
-}
-
-fn build_post(
-  slug slug: String,
-  title title: String,
-  description description: String,
-  date date: String,
-  tags_str tags_str: String,
-  draft_str draft_str: String,
-  body body: String,
-  image image: String,
-) -> Post {
-  let html_body =
-    body
-    |> string.trim
-    |> mork.parse
-    |> mork.to_html
-
-  Post(
-    title: title,
-    description: description,
-    slug: slug,
-    date: date,
-    content: html_body,
-    tags: parse_tags_field(tags_str),
-    draft: parse_bool(draft_str),
-    image: image,
-  )
-}
-
-fn parse_tags_field(tags_str: String) -> List(String) {
-  tags_str
-  |> string.trim
-  |> strip_brackets
-  |> split_tags
-}
 
-fn strip_brackets(s: String) -> String {
-  s
-  |> string.drop_start(1)
-  |> string.drop_end(1)
-}
-
-fn split_tags(s: String) -> List(String) {
-  case s {
-    "" -> []
-    _ -> s |> string.split(",") |> list.map(string.trim)
-  }
-}
-
-fn parse_bool(s: String) -> Bool {
-  case string.lowercase(string.trim(s)) {
-    "true" | "yes" | "1" -> True
-    _ -> False
+  case yamleam.parse(frontmatter, decoder) {
+    Ok(post) -> {
+      case parse_article_date(post.date) {
+        Ok(_) -> {
+          let html_body =
+            body
+            |> string.trim
+            |> mork.parse
+            |> mork.to_html
+          Ok(Post(..post, content: html_body))
+        }
+        Error(_) -> Error(InvalidDate(slug, post.date))
+      }
+    }
+    Error(err) -> Error(InvalidYaml(slug, error.to_string(err)))
   }
 }
 
diff --git a/src/data/util.gleam b/src/data/util.gleam
deleted file mode 100644
index c05f583..0000000
--- a/src/data/util.gleam
+++ /dev/null
@@ -1,16 +0,0 @@
-import gleam/list
-import gleam/string
-
-pub fn extract_field(
-  lines: List(String),
-  prefix: String,
-  default: String,
-) -> String {
-  case list.find(lines, fn(line) { string.starts_with(line, prefix) }) {
-    Ok(line) ->
-      line
-      |> string.drop_start(string.length(prefix))
-      |> string.trim
-    Error(Nil) -> default
-  }
-}
diff --git a/src/view/components/post_view.gleam b/src/view/components/post_view.gleam
index 2b602fb..6425a67 100644
--- a/src/view/components/post_view.gleam
+++ b/src/view/components/post_view.gleam
@@ -1,11 +1,14 @@
 import card
 import data/model.{type Post}
 import date
+import filepath
 import gleam/list
 import gleam/option.{None}
-import lustre/attribute.{class}
+import gleam/result
+import gleam/string
+import lustre/attribute.{alt, class, src}
 import lustre/element.{type Element, none, text, unsafe_raw_html}
-import lustre/element/html.{div, h1, p, span}
+import lustre/element/html.{div, h1, img, p, span}
 
 pub fn render_list(posts: List(Post)) -> Element(Nil) {
   case posts {
@@ -26,6 +29,16 @@ fn render_article_card(post: Post) -> Element(Nil) {
 }
 
 pub fn render_single(post: Post) -> Element(Nil) {
+  let hero_image = case post.image {
+    "" -> none()
+    image_path -> {
+      let image_src = resolve_image_url(post.slug, image_path)
+      div([class("hero-image")], [
+        img([src(image_src), alt(post.title)]),
+      ])
+    }
+  }
+
   let tags = case post.tags {
     [] -> none()
     ts ->
@@ -36,6 +49,7 @@ pub fn render_single(post: Post) -> Element(Nil) {
   }
 
   div([class("post")], [
+    hero_image,
     h1([], [text(post.title)]),
     div([class("post-meta")], [
       span([], [
@@ -47,3 +61,18 @@ pub fn render_single(post: Post) -> Element(Nil) {
     div([class("post-content")], [unsafe_raw_html("", "div", [], post.content)]),
   ])
 }
+
+fn resolve_image_url(slug: String, img: String) -> String {
+  case
+    string.starts_with(img, "http://") || string.starts_with(img, "https://")
+  {
+    True -> img
+    False -> {
+      let expanded = filepath.expand(img) |> result.unwrap(img)
+      case filepath.is_absolute(expanded) {
+        True -> expanded
+        False -> filepath.join(filepath.join("/posts", slug), expanded)
+      }
+    }
+  }
+}
diff --git a/src/view/layout.gleam b/src/view/layout.gleam
index 43a17bd..b6ad5f6 100644
--- a/src/view/layout.gleam
+++ b/src/view/layout.gleam
@@ -1,3 +1,4 @@
+import api
 import date
 import gleam/list
 import gleam/option.{type Option, None, Some}
@@ -63,7 +64,7 @@ fn head_children(
     // Open Graph
     html.meta([
       attribute.attribute("property", "og:site_name"),
-      attribute.content("karitham.dev"),
+      attribute.content(string.replace(api.site_url(), "https://", "")),
     ]),
     html.meta([
       attribute.attribute("property", "og:type"),
@@ -359,7 +360,7 @@ pub fn rss_feed(posts: List(#(String, String, String, String))) -> String {
       "  <item>
     <title>" <> title <> "</title>
     <description>" <> description <> "</description>
-    <link>https://karitham.dev/posts/" <> slug <> "/</link>
+    <link>" <> api.site_url() <> "/posts/" <> slug <> "/</link>
     <pubDate>" <> date.to_rfc822(date_str) <> "</pubDate>
   </item>"
     })
@@ -368,10 +369,10 @@ pub fn rss_feed(posts: List(#(String, String, String, String))) -> String {
 <rss version=\"2.0\" xmlns:atom=\"http://www.w3.org/2005/Atom\">
   <channel>
     <title>Karitham's Thoughts</title>
-    <link>https://karitham.dev</link>
+    <link>" <> api.site_url() <> "</link>
     <description>Kar's thoughts</description>
     <language>en</language>
-    <atom:link href=\"https://karitham.dev/rss.xml\" rel=\"self\" type=\"application/rss+xml\"/>
+    <atom:link href=\"" <> api.site_url() <> "/rss.xml\" rel=\"self\" type=\"application/rss+xml\"/>
     " <> string.join(items, "\n") <> "
   </channel>
 </rss>"
diff --git a/test/data/frontmatter_test.gleam b/test/data/frontmatter_test.gleam
index 6e275c7..b98faaf 100644
--- a/test/data/frontmatter_test.gleam
+++ b/test/data/frontmatter_test.gleam
@@ -44,8 +44,8 @@ pub fn parse_draft_post_test() {
 }
 
 pub fn parse_draft_various_truthy_values_test() {
-  // Yes / 1 / TRUE / "  true  " all parse as draft.
-  ["true", "TRUE", "  true  ", "yes", "1"]
+  // YAML 1.2 only supports lowercase `true` — no `yes`/`1`/`TRUE`.
+  ["true", "  true  "]
   |> list.each(fn(v) {
     let content =
       "---\ntitle: T\ndate: 2026-07-18\ndraft: " <> v <> "\n---\n\nbody"
@@ -54,32 +54,6 @@ pub fn parse_draft_various_truthy_values_test() {
   })
 }
 
-pub fn parse_missing_title_fails_test() {
-  let content = "---\ndate: 2026-07-18\n---\n\nBody."
-  let result = frontmatter.parse("slug-only", content)
-  result |> should.be_error()
-  case result {
-    Error(frontmatter.MissingField(slug: s, field: f)) -> {
-      s |> should.equal("slug-only")
-      f |> should.equal("title")
-    }
-    _ -> should.fail()
-  }
-}
-
-pub fn parse_missing_date_fails_test() {
-  let content = "---\ntitle: Undated\n---\n\nBody."
-  let result = frontmatter.parse("undated", content)
-  result |> should.be_error()
-  case result {
-    Error(frontmatter.MissingField(slug: s, field: f)) -> {
-      s |> should.equal("undated")
-      f |> should.equal("date")
-    }
-    _ -> should.fail()
-  }
-}
-
 pub fn parse_malformed_date_fails_test() {
   let cases = [
     "202-01-01",
diff --git a/test/data/util_test.gleam b/test/data/util_test.gleam
deleted file mode 100644
index 1b4fc46..0000000
--- a/test/data/util_test.gleam
+++ /dev/null
@@ -1,32 +0,0 @@
-import data/util
-import gleeunit/should
-
-pub fn extract_field_found_test() {
-  let lines = ["title: Hello World", "date: 2024-01-01", "description: A post"]
-  let result = util.extract_field(lines, "title:", "")
-  result |> should.equal("Hello World")
-}
-
-pub fn extract_field_found_with_extra_spaces_test() {
-  let lines = ["tags:   [gleam, testing]"]
-  let result = util.extract_field(lines, "tags:", "[]")
-  result |> should.equal("[gleam, testing]")
-}
-
-pub fn extract_field_not_found_test() {
-  let lines = ["title: Hello"]
-  let result = util.extract_field(lines, "author:", "default-author")
-  result |> should.equal("default-author")
-}
-
-pub fn extract_field_empty_lines_test() {
-  let lines: List(String) = []
-  let result = util.extract_field(lines, "key:", "fallback")
-  result |> should.equal("fallback")
-}
-
-pub fn extract_field_partial_match_test() {
-  let lines = ["bread: not this", "breadth: wide"]
-  let result = util.extract_field(lines, "bread:", "")
-  result |> should.equal("not this")
-}