diff --git a/excalidraw/excalidraw-app/app-jotai.ts b/excalidraw/excalidraw-app/app-jotai.ts
index c6ed38d..8f78840 100644
--- a/excalidraw/excalidraw-app/app-jotai.ts
+++ b/excalidraw/excalidraw-app/app-jotai.ts
@@ -48,7 +48,7 @@ const STORAGE_CONFIG_SESSION_STORAGE_KEY =
   "excalidraw-storage-config-credentials";
 
 const getInitialStorageConfig = (): StorageConfig => {
-  const defaultConfig: StorageConfig = { type: "default" };
+  const defaultConfig: StorageConfig = { type: "indexed-db" };
 
   try {
     const nonSensitive = localStorage.getItem(STORAGE_CONFIG_LOCAL_STORAGE_KEY);
diff --git a/excalidraw/excalidraw-app/components/StorageSettingsDialog.tsx b/excalidraw/excalidraw-app/components/StorageSettingsDialog.tsx
index ea42f3c..d78a4d5 100644
--- a/excalidraw/excalidraw-app/components/StorageSettingsDialog.tsx
+++ b/excalidraw/excalidraw-app/components/StorageSettingsDialog.tsx
@@ -9,8 +9,11 @@ import { storageConfigAtom } from "../app-jotai";
 export type StorageType = "default" | "kv" | "s3" | "indexed-db";
 
 const StorageSettingsDialog = ({ onClose }: { onClose: () => void }) => {
+  const storedToken = localStorage.getItem("token"); // 如果未登录，无法使用后端存储
   const [config, setConfig] = useAtom(storageConfigAtom);
-  const [storageType, setStorageType] = useState<StorageType>(config.type);
+  const [storageType, setStorageType] = useState<StorageType>(
+    !storedToken && config.type === "default" ? "indexed-db" : config.type,
+  );
 
   // Local state for form inputs
   const [kvUrl, setKvUrl] = useState(config.kvUrl || "");
@@ -96,8 +99,9 @@ const StorageSettingsDialog = ({ onClose }: { onClose: () => void }) => {
       default:
         return (
           <p>
-            Your data is stored on the default backend of this Excalidraw
-            instance. This requires you to be logged in.
+            {!storedToken
+              ? "You must be logged in to use the default backend storage. Please log in and try again."
+              : "Your data is stored on the default backend of this Excalidraw instance. This requires you to be logged in."}
           </p>
         );
     }
@@ -125,7 +129,15 @@ const StorageSettingsDialog = ({ onClose }: { onClose: () => void }) => {
           }}
         >
           <option value="indexed-db">Browser (IndexedDB)</option>
-          <option value="default">Default Backend (Online)</option>
+          <option
+            value="default"
+            disabled={!storedToken}
+            title={
+              !storedToken ? "Please log in to use this option" : undefined
+            }
+          >
+            Default Backend (Online)
+          </option>
           <option value="kv">Cloudflare KV (Online)</option>
           <option value="s3">Amazon S3 (Online)</option>
         </select>
diff --git a/excalidraw/excalidraw-app/data/storageAdapters/BackendStorageAdapter.ts b/excalidraw/excalidraw-app/data/storageAdapters/BackendStorageAdapter.ts
index a50a156..306d107 100644
--- a/excalidraw/excalidraw-app/data/storageAdapters/BackendStorageAdapter.ts
+++ b/excalidraw/excalidraw-app/data/storageAdapters/BackendStorageAdapter.ts
@@ -7,6 +7,7 @@ import {
 } from "../storage";
 import { nanoid } from "nanoid";
 import { jwtDecode } from "jwt-decode";
+import { generateThumbnail } from "../thumbnail";
 
 export class AuthError extends Error {
   constructor(message: string) {
@@ -64,19 +65,8 @@ export class BackendStorageAdapter implements IStorageAdapter {
       }
       throw new Error(`Failed to list canvases: ${response.statusText}`);
     }
-    // Backend doesn't send userId, so we enrich the data here.
-    const canvases: Omit<CanvasMetadata, "userId">[] = await response.json();
-    const token = localStorage.getItem("token");
-    if (!token) {
-      return [];
-    }
-    const userId = getUserIdFromJwt(token);
-    if (!userId) {
-      console.error("Could not determine userId from token.");
-      return [];
-    }
-
-    return canvases.map((canvas) => ({ ...canvas, userId }));
+    const canvases: CanvasMetadata[] = await response.json();
+    return canvases;
   }
 
   async loadCanvas(id: string): Promise<CanvasData | null> {
@@ -98,7 +88,21 @@ export class BackendStorageAdapter implements IStorageAdapter {
   }
 
   async saveCanvas(id: string, data: CanvasData): Promise<void> {
-    const saveData = dehydrateCanvasData(data);
+    let dataForUpload: CanvasData;
+    if (data.thumbnail) {
+      dataForUpload = data;
+    } else {
+      const thumbnail = await generateThumbnail(
+        data.elements,
+        data.appState,
+        data.files,
+      );
+      dataForUpload = {
+        ...data,
+        thumbnail: data.elements.length > 0 ? thumbnail : undefined,
+      };
+    }
+    const saveData = dehydrateCanvasData(dataForUpload);
 
     const response = await fetch(`${API_BASE_URL}/${id}`, {
       method: "PUT",
@@ -123,14 +127,24 @@ export class BackendStorageAdapter implements IStorageAdapter {
     if (!userId) {
       throw new Error("Could not parse user ID from token.");
     }
+    const thumbnail = await generateThumbnail(
+      data.elements,
+      data.appState,
+      data.files,
+    );
+    const dataWithThumbnail: CanvasData = {
+      ...data,
+      thumbnail: data.elements.length > 0 ? thumbnail : undefined,
+    };
 
-    await this.saveCanvas(newId, data);
+    await this.saveCanvas(newId, dataWithThumbnail);
     return {
       id: newId,
       name: data.appState?.name || "Untitled",
       createdAt: new Date().toISOString(),
       updatedAt: new Date().toISOString(),
       userId,
+      thumbnail: dataWithThumbnail.thumbnail,
     };
   }