Video thumbnails

I got a WMD to store my daughter’s video files as we’d run out of space on the iPad.  Because they’re all Pixar / Disney / Dreamworks films the thumbails are all identical and my daugher can’t choose the film she’s looking for.

Whilst I wait (hope?) that the WMD software will support more control over video thumbnails, in the meantime can anyone recommend how to “inject” an image as the first frame of an MP4 without recoding the entire film?

Many thanks