If you want to download an thumbnail from a YouTube video, there’s two ways to do it. The first way works if you’re downloading a thumbnail from your own video. However, if you want to download a thumbnail from someone else’s YouTube video, just use this URL, but replace “youtube video id” with the ID of the YouTube video whose thumbnail you want to download. Remember that when you download someone else’s thumbnail you’re downloading their copyrighted material. Use it only in a Fair Use case.