diff --git a/.github/workflows/check-mod.yml b/.github/workflows/check-mod.yml
index ddcf523a..06c1c26f 100644
--- a/.github/workflows/check-mod.yml
+++ b/.github/workflows/check-mod.yml
@@ -41,7 +41,7 @@ jobs:
           for dir in mods/*/; do
             if [ -d "$dir" ]; then
               MOD_DIR="$(basename "$dir")"
-              THUMBNAIL="$dir/thumbnail.png"
+              THUMBNAIL="$dir/thumbnail.jpg"
 
               if [ -f "$THUMBNAIL" ]; then
                 # Extract width and height using ImageMagick
diff --git a/README.md b/README.md
index 82bb28d0..295421c4 100644
--- a/README.md
+++ b/README.md
@@ -45,7 +45,7 @@ This file stores essential metadata in JSON format. **Make sure you adhere to th
 - **repo**: A link to your mod’s repository.
 - **downloadURL**: A direct link to the latest version of your released mod. (Can be same as `repo` if no separate download link exists.)
 
-### 3. thumbnail.png (Optional)
+### 3. thumbnail.jpg (Optional)
 If included, this image will appear alongside your mod in the index. Maximum and recommended size is 1920 × 500 pixels.
 
 ## How to Publish