Each YouTube video has 4 generated images. They are predictably formatted as follows:
- 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).
http://img.youtube.com/vi/
/default.jpg
high quality version of the thumbnail
http://img.youtube.com/vi//hqdefault.jpg
medium quality version of the thumbnail
http://img.youtube.com/vi//mqdefault.jpg
standard definition version of the thumbnail
http://img.youtube.com/vi//sddefault.jpg
maximum resolution version of the thumbnail
http://img.youtube.com/vi//maxresdefault.jpg
Example:- https://www.youtube.com/watch?v=pKtAaKid4CM
http://img.youtube.com/vi//0.jpg
YouTube Video Id is :- pKtAaKid4CM
http://img.youtube.com/vi/pKtAaKid4CM/0.jpg