Rating between PG-13 and R

A picture of a woman in a tame one piece swimsuit is R (it’s so rated towards R, even a woman in full body wetsuit that hides her curves gets an R by default). A picture of a woman in a bra and panties (no matter how not skimpy or skimpy) gets rated R. A picture of the same woman without a bra, showing her nipples, is… also R. Clearly this suboptimal behavior. As dropping the first two types of images to PG-13 might not be desired (even if plenty of movies with PG-13 ratings have extensive underwear scenes), the obvious and only solution is to add an additional rating between the two.