Download HD thumbnails from any YouTube video — 100% free, runs in your browser
Thumbnails are useful for competitor research, for building a channel mood board, or for recovering your own thumbnail when you have lost the original file. This fetches every resolution YouTube publishes.
maxresdefault at 1280 by 720 is the highest. Not every video has it — older or low-resolution uploads may only offer smaller sizes.
For study and inspiration, yes. Republishing it as your own is copyright infringement.
Downloading a publicly served image is fine. These are the same URLs YouTube uses to display thumbnails everywhere.