For the high quality version of the thumbnail use a url similar to this:
https://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:
https://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:
https://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:
https://img.youtube.com/vi/<insert-youtube-video-id-here>/maxresdefault.jpg