Sounds like what you're after is an image gallery. I see www.hotscripts.com cited often in these forums when someone asks about one of these, but I'm sure a Google search for PHP image gallery would turn up plenty of other sources.
I'm guessing that the only thing missing would be a one-off script that will take all the existing images, generate thumbnails, and register all of them in the database. There may be some that do bulk updates, however.