Hi!
Is Justin's suggestion of aliases feasible, so that "filetype:image" is the same as "filetype:bitmap"? there's still the question of what to map to what, but that seems like it would help if it's easy enough to implement. And of course a histogram of filetype:<x> would tell you what people are trying to use.
We could have a config that says "translate image, picture, photo, etc to bitmap" - shouldn't be too hard to do, just add config and translate when processing the item. I don't see much issue with that. Of course, the config should be sane otherwise it becomes a mess but we can make a good one I suppose.