I have no idea what the details of the right answer are, or how to go about getting to that point. But I agree that some distinctions need to be made. Given the free form nature of the site; perhaps using #nsfw-art when it applies would be a start. Of course, different people have different definitions. But if it stayed relatively small, we might be able to avoid having it flooded by random porn, at least. The other distinctions can be handled by separate tags, I suppose.