How to get the url of Youtube video cover image: Just copy the cover image address below, and then replace the video ID inside with the target video ID.
//Default cover image
http://img.youtube.com/vi/<insert-youtube-video-id-here>/default.jpg
//Standard definition cover image
http://img.youtube.com/vi/<insert-youtube-video-id-here>/sddefault.jpg
//Medium definition cover image
http://img.youtube.com/vi/<insert-youtube-video-id-here>/mqdefault.jpg
//HD cover image
http://img.youtube.com/vi/<insert-youtube-video-id-here>/hqdefault.jpg
//Maximum definition cover image
http://img.youtube.com/vi/<insert-youtube-video-id-here>/maxresdefault.jpg