Grabbing a YouTube video thumbnail is easier than you’d think. YouTube uses a standard naming convention for it’s screenshot images, just replace %VIDEO-ID% with a YouTube video ID in the format below and link directly to the image.

http://img.youtube.com/vi/%VIDEO-ID%/0.jpg
http://img.youtube.com/vi/%VIDEO-ID%/1.jpg
http://img.youtube.com/vi/%VIDEO-ID%/2.jpg
http://img.youtube.com/vi/%VIDEO-ID%/3.jpg

0.jpg is full size, while others are scaled thumbnails. There are also other sizes available for the default image that is being used for the video.

Default quality

http://img.youtube.com/vi/%VIDEO-ID%/default.jpg

High quality

http://img.youtube.com/vi/%VIDEO-ID%/hqdefault.jpg

Medium quality

http://img.youtube.com/vi/%VIDEO-ID%/mqdefault.jpg

Standard definition

http://img.youtube.com/vi/%VIDEO-ID%/sddefault.jpg

Highest resolution

http://img.youtube.com/vi/%VIDEO-ID%/maxresdefault.jpg

All of these are available over https:// as well, just swap out http to https. You can also use the shorthand domain i3.ytimg.com instead of img.youtube.com if you’d like to shorten things up.