[Plugin: Simple Flash Video] Thumbs

Hello, thank you for the excellent plugin. I am on 1.7 version. For some reason it displays mp4 clips but is not creating the thumbnails (not always). Is there a way to add specific thumbs proper to the video in question manually? I mean, not using the ‘across the board’ solution of “default solution”.

http://wordpress.org/extend/plugins/simple-flash-video/