Your custom thumbnail image should be as large as possible. It will be used as the preview image in the embedded player. We recommend your custom thumbnails:

Have a resolution of 1280x720 (with minimum width of 640 pixels).
Be uploaded in image formats such as JPG, GIF, or PNG.
Remain under the 2MB limit.
Try to use a 16:9 aspect ratio as it's the most used in YouTube players and previews.