Woman has a third nipple... on her foot.

It's not a sexual picture. Obscenity is (implied) sexuality. Nipples aren't inherently sexual, it's all about the intent of the poster. This picture is... quite non-sexual.

Unless you have a foot fetish. Then this is a double win.

Obscenity: Graphic/Suggestive Imagery
Definition: Any human or anthropomorphic representation, real or imaginary, posted in any degree of undress, including but not limited to bathing suits, or in a manner that sexualizes or eroticizes them.

You can't post breasts/nipples even if they are not sexualized.