All YouTube videos have four default images.
They have standard formats :
http://img.youtube.com/vi/video-id/0.jpg
http://img.youtube.com/vi/video-id/1.jpg
http://img.youtube.com/vi/video-id/2.jpg
http://img.youtube.com/vi/video-id/3.jpg
0.jpg in the list is a full size image and rest of them are thumbnail images( 1.jpg, 2.jpg, 3.jpg).
Also have some additional sizes but the availability of following images are depends on the video.
http://img.youtube.com/vi/video-id/default.jpg
http://img.youtube.com/vi/video-id/hqdefault.jpg -high quality format of the thumbnail
http://img.youtube.com/vi/video-id/mqdefault.jpg – medium quality format of the thumbnail
http://img.youtube.com/vi/video-id/sddefault.jpg – standard definition format of the thumbnail
http://img.youtube.com/vi/video-id/maxresdefault.jpg – maximum resolution format of the thumbnail