diff --git a/next.config.ts b/next.config.ts
index ceb8aa2..f1d322b 100644
--- a/next.config.ts
+++ b/next.config.ts
@@ -3,6 +3,25 @@ import type { NextConfig } from "next";
 const nextConfig: NextConfig = {
   allowedDevOrigins: ["192.168.100.6"],
   output: "standalone",
+  // Files in public/ are served with `Cache-Control: public, max-age=0` by
+  // default, because Next.js can't know whether they change. kd-tile.png is
+  // hot-linked by an external HTML email signature, so its URL is a fixed
+  // contract and its bytes never change — pin it to a long-lived immutable
+  // cache so mail clients and their image proxies stop re-fetching it.
+  // Headers are matched before the filesystem, so this applies to public/.
+  async headers() {
+    return [
+      {
+        source: "/kd-tile.png",
+        headers: [
+          {
+            key: "Cache-Control",
+            value: "public, max-age=31536000, immutable",
+          },
+        ],
+      },
+    ];
+  },
 };
 
 export default nextConfig;
diff --git a/public/kd-tile-big.png b/public/kd-tile-big.png
new file mode 100644
index 0000000..bd2579b
Binary files /dev/null and b/public/kd-tile-big.png differ
diff --git a/public/kd-tile.png b/public/kd-tile.png
index bd2579b..8c2d523 100644
Binary files a/public/kd-tile.png and b/public/kd-tile.png differ