diff --git a/publish/src/core/markdown.ts b/publish/src/core/markdown.ts
new file mode 100644
index 0000000..69bab2a
--- /dev/null
+++ b/publish/src/core/markdown.ts
@@ -0,0 +1,29 @@
+const IMG_RE = /!\[([^\]]*)\]\(([^)\s]+)(?:\s+=\d+x\d+)?\)/g
+
+function isAbsolute(url: string): boolean {
+  return /^(https?:)?\/\//i.test(url)
+}
+
+export function rewriteImageUrls(md: string, mapping: Map<string, string>): string {
+  return md.replace(IMG_RE, (full, alt: string, url: string) => {
+    if (isAbsolute(url)) return full.replace(/\s+=\d+x\d+\)$/, ')')
+    let decoded: string
+    try {
+      decoded = decodeURIComponent(url)
+    } catch {
+      decoded = url
+    }
+    const target = mapping.get(decoded) ?? mapping.get(url)
+    if (!target) return full.replace(/\s+=\d+x\d+\)$/, ')')
+    return `![${alt}](${target})`
+  })
+}
+
+export function resolveCoverUrl(
+  coverRaw: string | undefined,
+  mapping: Map<string, string>,
+): string | undefined {
+  if (!coverRaw) return undefined
+  if (isAbsolute(coverRaw)) return coverRaw
+  return mapping.get(coverRaw)
+}
diff --git a/publish/tests/markdown_test.ts b/publish/tests/markdown_test.ts
new file mode 100644
index 0000000..d07bf86
--- /dev/null
+++ b/publish/tests/markdown_test.ts
@@ -0,0 +1,53 @@
+import { assertEquals } from '@std/assert'
+import { rewriteImageUrls } from '../src/core/markdown.ts'
+
+Deno.test('rewriteImageUrls: ersetzt ![alt](file) durch Mapping', () => {
+  const mapping = new Map([['cat.png', 'https://blossom.example/hash.png']])
+  const input = '![cat](cat.png)'
+  assertEquals(rewriteImageUrls(input, mapping), '![cat](https://blossom.example/hash.png)')
+})
+
+Deno.test('rewriteImageUrls: absolute URL bleibt unverändert', () => {
+  const mapping = new Map([['cat.png', 'https://blossom.example/hash.png']])
+  const input = '![cat](https://other.com/cat.png)'
+  assertEquals(rewriteImageUrls(input, mapping), input)
+})
+
+Deno.test('rewriteImageUrls: entfernt =WxH-Suffix', () => {
+  const mapping = new Map([['cat.png', 'https://blossom.example/hash.png']])
+  const input = '![cat](cat.png =300x200)'
+  assertEquals(rewriteImageUrls(input, mapping), '![cat](https://blossom.example/hash.png)')
+})
+
+Deno.test('rewriteImageUrls: bild-in-link [![alt](file)](link)', () => {
+  const mapping = new Map([['cat.png', 'https://blossom.example/hash.png']])
+  const input = '[![cat](cat.png)](https://target.example.com)'
+  assertEquals(
+    rewriteImageUrls(input, mapping),
+    '[![cat](https://blossom.example/hash.png)](https://target.example.com)',
+  )
+})
+
+Deno.test('rewriteImageUrls: mehrere Bilder im Text', () => {
+  const mapping = new Map([
+    ['a.png', 'https://bl/a-hash.png'],
+    ['b.jpg', 'https://bl/b-hash.jpg'],
+  ])
+  const input = 'Text ![a](a.png) more ![b](b.jpg) end'
+  assertEquals(
+    rewriteImageUrls(input, mapping),
+    'Text ![a](https://bl/a-hash.png) more ![b](https://bl/b-hash.jpg) end',
+  )
+})
+
+Deno.test('rewriteImageUrls: lässt unbekannte Dateinamen stehen', () => {
+  const mapping = new Map([['cat.png', 'https://bl/c.png']])
+  const input = '![x](missing.jpg)'
+  assertEquals(rewriteImageUrls(input, mapping), input)
+})
+
+Deno.test('rewriteImageUrls: URL-Dekodierung für Leerzeichen-Namen', () => {
+  const mapping = new Map([['file with spaces.png', 'https://bl/hash.png']])
+  const input = '![x](file%20with%20spaces.png)'
+  assertEquals(rewriteImageUrls(input, mapping), '![x](https://bl/hash.png)')
+})