Thumbnails are an important part of your YouTube videos because it's the first thing users see besides the title. YouTube automatically generates three thumbnails for your uploaded video, and you can't submit your own thumbnail, but you can view large versions of the thumbnails they choose, in JPG format. The key is add "img" before the video's URL, and to add the thumbnail number and "jpg" at the end. You can also see the current thumbnail being used. This is great if you want to use someone else's thumbnail… because you can say anybody's thumbnails!
For more information, visit the 4blogger post.