In fact, in the UK and some other places what is illegal is "indecent" images of someone under 18, not nudity or sexual content in and of itself. So the sexual pose would likely be a problem whether or not the nipples are exposed. In the UK for the lowest severity images the context the pictures are found in also matters - even a picture that is ok in a family album (e.g. an innocent picture of a child in the bath) would not be ok if found in a collection of pictures clearly intended for sexual gratification.
