Do you want a new folder to appear with every download or just automatically put into a category?

That doesn't make sense why you would want that (why do you want that?); but that could probably be done through a simple hack which works by loading all the folder names into an array, which in turn cycles out putting all the pictures into each folder.

I wouldn't bother trying to find a plugin for that (but then I hate wordpress and prefer to code all my own stuff), so maybe there is something out there that does this. Although it seems like a rather specific request, so I wouldn't know for sure.