We had a thread on this a while ago, but I don't see it now. It's not really that enforced, but it's there to be able to call people on the rule. Really, we're not supposed to upload, say, the virtual paper doll sets from dating sims, because they're all almost the same damn image, and it would basically be a flood of the same thing over and over.
The CGs, on the other hand, actually being, well, unique, seem to be acceptable (if for no other reason than people find it posted elsewhere and don't realize that it's a game CG; they didn't mean to break the rules then). Just don't flood with them.