This is a timely question, I was just wondering the same thing myself. I'm working on a 3.1 Tiki site, that was previously upgraded from 2.x, all with image galleries (no file galleries used). Now, I'm looking at "converting" them to file galleries in preparation for the future change but will run into the problem of broken image links. AFAIK, there is no way to say "move image X from image-gallery Y into file-gallery Z". That would be very handy though!