OK, so I fixed the bug that made each title overwrite others with the same text. Now every possible combination of parameters gets its own subdirectory on which to store the images.
Now I only have to work out how to get MT to tell me where to store the files without having the user specifying the path manually (which can be daunting for some shared hosting users). I have a couple of ideas already.
It’s being interesting to use Subversion, BTW