By Default Each YouTube video has 4 generated images, format can be given as follows

Syntax/Format

http://img.youtube.com/vi/<insert-youtube-video-id-here>/0.jpg
http://img.youtube.com/vi/<insert-youtube-video-id-here>/1.jpg
http://img.youtube.com/vi/<insert-youtube-video-id-here>/2.jpg
http://img.youtube.com/vi/<insert-youtube-video-id-here>/3.jpg

NOTE: The first one in the list is a full size image and others are thumbnail images.

Here is default image,
http://img.youtube.com/vi/<insert-youtube-video-id-here>/default.jpg

For the high quality version of the thumbnail use a url similar to this:

http://img.youtube.com/vi/<insert-youtube-video-id-here>/hqdefault.jpg

There is also a medium quality version of the thumbnail, using a url similar to the HQ:

http://img.youtube.com/vi/<insert-youtube-video-id-here>/mqdefault.jpg

For the standard definition version of the thumbnail, use a url similar to this:

http://img.youtube.com/vi/<insert-youtube-video-id-here>/sddefault.jpg

For the maximum resolution version of the thumbnail use a url similar to this:

http://img.youtube.com/vi/<insert-youtube-video-id-here>/maxresdefault.jpg

NOTE: Above URL works both on https & http