Replace <youtube-video-id-here> with Youtube video id
Default Thumbnail
http://img.youtube.com/vi/<youtube-video-id-here>/default.jpg
High Quality Thumbnail
http://img.youtube.com/vi/<youtube-video-id-here>/hqdefault.jpg
Medium Quality
http://img.youtube.com/vi/<youtube-video-id-here>/mqdefault.jpg
Standard Definition
http://img.youtube.com/vi/<youtube-video-id-here>/sddefault.jpg
Maximum Resolution
http://img.youtube.com/vi/<youtube-video-id-here>/maxresdefault.jpg
Default Thumbnail
http://img.youtube.com/vi/<youtube-video-id-here>/default.jpg
High Quality Thumbnail
http://img.youtube.com/vi/<youtube-video-id-here>/hqdefault.jpg
Medium Quality
http://img.youtube.com/vi/<youtube-video-id-here>/mqdefault.jpg
Standard Definition
http://img.youtube.com/vi/<youtube-video-id-here>/sddefault.jpg
Maximum Resolution
http://img.youtube.com/vi/<youtube-video-id-here>/maxresdefault.jpg