docs: media thumbnail as Instagram Reel cover (--json media objects)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

FEATURES.md

@@ -227,7 +227,7 @@ postiz posts:create --json complex-post.json
         id: string,
         path: string,
         alt?: string,
-        thumbnail?: string
+        thumbnail?: string   // Uploaded JPEG/PNG path used as video thumbnail / cover (e.g. Instagram Reel cover); needs --json
       }>,
       delay?: number,        // Minutes
       id?: string

PROVIDER_SETTINGS.md

@@ -178,6 +178,15 @@ postiz posts:create \
   -i "instagram-123"
 ```
 
+**Reel Cover Image:** to set a custom Reel cover, pass the video as a media
+object with a `thumbnail` (uploaded JPEG/PNG path) using `--json`:
+```json
+"image": [
+  { "id": "reel", "path": "<uploaded video path>", "thumbnail": "<uploaded cover.jpg path>" }
+]
+```
+Without `thumbnail`, Instagram picks a frame from the video as the cover.
+
 **Story Example:**
 ```bash
 postiz posts:create \

SKILL.md

@@ -353,6 +353,10 @@ postiz posts:create \
   -s "2024-12-31T12:00:00Z" \
   -m "$VIDEO_PATH" \
   -i "tiktok-id"
+
+# Use the uploaded image as the video thumbnail / Instagram Reel cover
+# (media objects with a thumbnail need the --json form):
+#   "image": [{ "id": "reel", "path": "$VIDEO_PATH", "thumbnail": "$THUMB_PATH" }]
 ```
 
 ### Pattern 3: Twitter Thread