Stackoverflow answer link - https://stackoverflow.com/a/2068371/867451
Copied the content for quick reference
Each YouTube video has four generated images. They are predictably formatted as follows:
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
The first one in the list is a full size image and others are thumbnail images. The default thumbnail image (i.e., one of 1.jpg, 2.jpg, 3.jpg) is:
https://img.youtube.com/vi/<insert-youtube-video-id-here>/default.jpg
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
All of the above URLs are available over HTTP too. Additionally, the slightly shorter hostname i3.ytimg.com works in place of img.youtube.com in the example URLs above.