Pulling Thumbs for Videos

I’m looking for some suggestions on how to pull thumbs for author posts. All the posts are for videos from youtube and vimeo and users use a front end plugin for posting them on my site. I don’t want to use the user avatar for the thumb but I can’t find a good way to auto add the actual thumb for the video. I know how to do it manually but that’s not going to be efficient to have to explain to all my users on how to add the url for the thumb.

Any suggestions are appreciated.