Haven't seen this here, maybe its useful for someone.
Using this URL (with the video ID) you can download the full rez thumbnail from any YouTube video!
https://img.youtube.com/vi/<insert-youtube-video-id-here>/maxresdefault.jpg
Using this URL (with the video ID) you can download the full rez thumbnail from any YouTube video!
https://img.youtube.com/vi/<insert-youtube-video-id-here>/maxresdefault.jpg