I can only restate their TOS
Our policy regarding adult or mature content may include any material that is not appropriate for all audiences. While this obviously includes full nudity or sexual activity, it may also include textually explicit sexual content, image or video content containing lewd or provocative poses, strategically covered nudity, see-through or sheer clothing, and close-ups of anatomy that would be inappropriate if shown nude.
I can't define what they consider lewd. But my guess would be such images fall under their criteria of lewd. You can use OpenX to serve up your own ads.