Each of every image has 4 category like

https://img.youtube.com/vi/insert-youtube-video-id-here/0.jpg

https://img.youtube.com/vi/insert-youtube-video-id-here/1.jpg

https://img.youtube.com/vi/insert-youtube-video-id-here/2.jpg

https://img.youtube.com/vi/insert-youtube-video-id-here/3.jpg 

Also you can get more image size by this

-- For default

https://img.youtube.com/vi/insert-youtube-video-id-here/default.jpg

-- For high quality

https://img.youtube.com/vi/insert-youtube-video-id-here/hqdefault.jpg 

-- For medium quality

https://img.youtube.com/vi/insert-youtube-video-id-here/mqdefault.jpg

-- For max size

https://img.youtube.com/vi/insert-youtube-video-id-here/maxresdefault.jpg