What is the best way to add custom thumbnails (user uploads thumbnail) for non youtube videos?

What is the best metholodgy and easiest way to add an ability to upload your own thumbnail if you add a non-youtube video, say an .mv4? BTW, although the media gallery module dosn't support playing it directly, it works perfectly witht he mediafront module and plays .mv4, mp2, mov etc from the media and media gallery module perfectly. I just need to know how to enable a custom thumbnail?