diff --git a/app/main.py b/app/main.py
index ad74651..07313c1 100644
--- a/app/main.py
+++ b/app/main.py
@@ -1044,17 +1044,27 @@ def save_image(upload: UploadFile, destination_dir: str, url_prefix: str):
             if image.width * image.height > MAX_IMAGE_PIXELS:
                 raise ValueError("Bildauflösung zu groß")
             image.thumbnail((2400, 2400))
+            has_alpha = image.mode in {"RGBA", "LA"} or (
+                image.mode == "P" and "transparency" in image.info
+            )
             if image.mode not in {"RGB", "RGBA"}:
-                image = image.convert("RGBA" if "transparency" in image.info else "RGB")
+                image = image.convert("RGBA" if has_alpha else "RGB")
             output = BytesIO()
-            image.save(output, format="WEBP", quality=88, method=6)
+            # Nicht jedes Browser-/Proxy-Setup verarbeitet serverseitig
+            # erzeugte WebP-Dateien zuverlässig. Nach der Validierung bleiben
+            # transparente Bilder PNG, normale Bilder werden als JPEG gespeichert.
+            output_format = "PNG" if image.mode == "RGBA" else "JPEG"
+            if output_format == "PNG":
+                image.save(output, format=output_format, optimize=True)
+            else:
+                image.save(output, format=output_format, quality=88, optimize=True)
             safe_contents = output.getvalue()
     except (ValueError, OSError, UnidentifiedImageError, Image.DecompressionBombError):
         return None, HTMLResponse(
             "Die Datei ist kein gültiges oder unterstütztes Bild.", status_code=400
         )
 
-    filename = str(uuid.uuid4()) + ".webp"
+    filename = str(uuid.uuid4()) + (".png" if output_format == "PNG" else ".jpg")
     destination = os.path.join(destination_dir, filename)
     with open(destination, "wb") as file:
         file.write(safe_contents)
diff --git a/app/templates/edit_concert.html b/app/templates/edit_concert.html
index 67e6330..bc26525 100644
--- a/app/templates/edit_concert.html
+++ b/app/templates/edit_concert.html
@@ -133,6 +133,45 @@
 </main>
 {% if can_edit_details %}
 <script>
+const editConcertForm = document.querySelector('form[enctype="multipart/form-data"]');
+const editFlyerInput = editConcertForm?.querySelector('input[name="flyer"]');
+async function optimizeEditFlyer(file) {
+    const bitmap = await createImageBitmap(file);
+    const maxSide = 1800;
+    const scale = Math.min(1, maxSide / Math.max(bitmap.width, bitmap.height));
+    const canvas = document.createElement("canvas");
+    canvas.width = Math.max(1, Math.round(bitmap.width * scale));
+    canvas.height = Math.max(1, Math.round(bitmap.height * scale));
+    canvas.getContext("2d").drawImage(bitmap, 0, 0, canvas.width, canvas.height);
+    bitmap.close();
+    let quality = 0.86;
+    let blob;
+    do {
+        blob = await new Promise(resolve => canvas.toBlob(resolve, "image/jpeg", quality));
+        quality -= 0.08;
+    } while (blob && blob.size > 900 * 1024 && quality >= 0.5);
+    if (!blob) throw new Error("Bild konnte nicht verarbeitet werden");
+    return new File([blob], "flyer.jpg", {type: "image/jpeg"});
+}
+
+editConcertForm?.addEventListener("submit", async event => {
+    if (editConcertForm.dataset.flyerOptimized === "true") return;
+    const file = editFlyerInput?.files[0];
+    if (!file) return;
+    event.preventDefault();
+    try {
+        const transfer = new DataTransfer();
+        transfer.items.add(await optimizeEditFlyer(file));
+        editFlyerInput.files = transfer.files;
+        editConcertForm.dataset.flyerOptimized = "true";
+        editConcertForm.requestSubmit();
+    } catch (error) {
+        console.error("Flyer optimization failed:", error);
+        editConcertForm.dataset.flyerOptimized = "true";
+        editConcertForm.requestSubmit();
+    }
+});
+
 const eventType = document.getElementById("event-type");
 const parentEventField = document.getElementById("parent-event-field");
 const parentEventId = document.getElementById("parent-event-id");
diff --git a/app/templates/new_concert.html b/app/templates/new_concert.html
index de7c35e..95e0640 100644
--- a/app/templates/new_concert.html
+++ b/app/templates/new_concert.html
@@ -877,6 +877,48 @@ flyerInput.addEventListener(
     }
 );
 
+// Große Flyer vor dem Absenden verkleinern. Dadurch bleibt der Multipart-
+// Request unter typischen Proxy-Limits und der Server muss keine riesigen
+// Originaldateien verarbeiten.
+const concertForm = flyerInput.form;
+async function optimizeFlyerUpload(file) {
+    const bitmap = await createImageBitmap(file);
+    const maxSide = 1800;
+    const scale = Math.min(1, maxSide / Math.max(bitmap.width, bitmap.height));
+    const canvas = document.createElement("canvas");
+    canvas.width = Math.max(1, Math.round(bitmap.width * scale));
+    canvas.height = Math.max(1, Math.round(bitmap.height * scale));
+    const context = canvas.getContext("2d");
+    context.drawImage(bitmap, 0, 0, canvas.width, canvas.height);
+    bitmap.close();
+    let quality = 0.86;
+    let blob;
+    do {
+        blob = await new Promise(resolve => canvas.toBlob(resolve, "image/jpeg", quality));
+        quality -= 0.08;
+    } while (blob && blob.size > 900 * 1024 && quality >= 0.5);
+    if (!blob) throw new Error("Bild konnte nicht verarbeitet werden");
+    return new File([blob], "flyer.jpg", {type: "image/jpeg"});
+}
+
+concertForm.addEventListener("submit", async event => {
+    if (concertForm.dataset.flyerOptimized === "true") return;
+    const file = flyerInput.files[0];
+    if (!file) return;
+    event.preventDefault();
+    try {
+        const transfer = new DataTransfer();
+        transfer.items.add(await optimizeFlyerUpload(file));
+        flyerInput.files = transfer.files;
+        concertForm.dataset.flyerOptimized = "true";
+        concertForm.requestSubmit();
+    } catch (error) {
+        console.error("Flyer optimization failed:", error);
+        concertForm.dataset.flyerOptimized = "true";
+        concertForm.requestSubmit();
+    }
+});
+
 
 /* ============================================================
    HTML ESCAPING
diff --git a/app/templates/profile.html b/app/templates/profile.html
index f30cb77..23ddb18 100644
--- a/app/templates/profile.html
+++ b/app/templates/profile.html
@@ -213,10 +213,10 @@ if (profileForm) {
             context.imageSmoothingQuality = "high";
             context.drawImage(bitmap, 0, 0, canvas.width, canvas.height);
             bitmap.close();
-            const blob = await new Promise(resolve => canvas.toBlob(resolve, "image/webp", .86));
+            const blob = await new Promise(resolve => canvas.toBlob(resolve, "image/jpeg", .88));
             if (!blob) throw new Error("Bild konnte nicht verarbeitet werden");
             const transfer = new DataTransfer();
-            transfer.items.add(new File([blob], "profilbild.webp", {type: "image/webp"}));
+            transfer.items.add(new File([blob], "profilbild.jpg", {type: "image/jpeg"}));
             input.files = transfer.files;
             profileForm.dataset.optimized = "true";
             status.textContent = `Optimiert: ${Math.ceil(blob.size / 1024)} KB – wird hochgeladen …`;