diff --git a/src/assets.ts b/src/assets.ts
index 62d70d7..5cad1d1 100644
--- a/src/assets.ts
+++ b/src/assets.ts
@@ -136,6 +136,14 @@ export async function getOrUploadImage(
 	log: Logger
 ): Promise<AssetInfo | undefined> {
 	const resolved = resolveUrl(base, url);
+
+	// Skip formats that image processors can't handle (e.g. ICO favicons)
+	const ext = extname(new URL(resolved, 'https://example.com').pathname).toLowerCase();
+	if (ext === '.ico') {
+		log.debug(`skipping unsupported image format: ${resolved}`);
+		return undefined;
+	}
+
 	const cached = stateAssets[resolved];
 	if (cached) {
 		log.debug(`reusing uploaded blob for ${resolved}`);