Use directories

Copying isn't necessary: if moving the physical location of files *requires* a namechange the naming system is broken. It's purpose is to abstract physical location away. If it can't do that, it isn't working properly.

Categorizing images into folders using hiearchial directories should work fine. If you use symlinks or bind mounts you'll get full device abstraction and even just union mounting gets you some abstraction (though it may pose some limits on the storage of labels (i.e. names of folders)). Union mounting may simplify management if the set of folders is quite static.