Each YouTube video has 4 generated images, like:
http://img.youtube.com/vi//0.jpg
http://img.youtube.com/vi//1.jpg
http://img.youtube.com/vi//2.jpg
http://img.youtube.com/vi//3.jpg

The first one in the list is a full size image and others are thumbnail images. The default thumbnail image (ie. one of 1.jpg, 2.jpg, 3.jpg) is:
http://img.youtube.com/vi//default.jpg

For the high quality version of the thumbnail use a url similar to this:
http://img.youtube.com/vi//hqdefault.jpg

Alternatively, you can use the YouTube API to get thumbnail images.