This is an in-case-you-didn’t-already-know tip, and is probably common knowledge to anyone who started writing YouTube scripts after v3 of the YouTube API came out. The gist is that in addition to the tiny thumbnail images that have always been available at a url containing the video id, you can also get a range of qualities and sizes.
This is a companion discussion topic for the original entry at https://brettterpstra.com/2016/02/09/better-youtube-thumbnail-images