Good if you need to extract the ^^ for some reason. replace $v with your video id.

raw links

https://img.youtube.com/vi/$v/default.jpg

https://img.youtube.com/vi/$v/hqdefault.jpg

max size

https://img.youtube.com/vi/$v/maxresdefault.jpg

Small auto-generated thumbnail 1

http://img.youtube.com/vi/$v/1.jpg

Small auto-generated thumbnail 2

http://img.youtube.com/vi/$v/2.jpg

Small auto-generated thumbnail 3

http://img.youtube.com/vi/$v/3.jpg

Small custom thumbnail

http://img.youtube.com/vi/$v/mqdefault.jpg

Medium custom thumbnail at 4:3 ratio

http://img.youtube.com/vi/$v/0.jpg

Large custom thumbnail

http://img.youtube.com/vi/$v/maxresdefault.jpg (only available if video has 720p or 1080p option)