I use Auto-Movie-Thumbnailer ( http://www.amt.cc ). The interface is a bit clunky, but with a little trial-and-error one can get it to work out fine. It generates .jpg thumbnails named the same as the video files it processes. It can handle different file types (.mp4, .avi, .wmv) within the folder it's thumbnailing.
I wish everyone who has more than, say, five video files in a torrent would use and auto-thumbnailer, whether this one or another..