You either need to calculate the 1/2 way point of the video clip to create the thumbnail or give the user an option to choose how far into the clip they want the thumbnail to be generated from (using a % of length / not absolute time) or both.

Right now, the thumbnails are generated from the first frame of video and most of the time you end up getting black or a logo if it's a movie.  Kind of silly if you ask me.