Which URLs are supported?

youtube.com/watch, /shorts/, /live/, /embed/, and youtu.be links — plus raw video IDs.

What is the highest quality available?

maxresdefault (1280×720) when the uploader provided it; otherwise SD/HQ fallbacks still work.

Can I reuse thumbnails I download?

Only with permission — thumbnails belong to the video owner. Use them for reference, analysis, or your own videos.