Hm. I know there are commercial products out there that do this, but the idea has always seemed pretty impossible to me.

Not so much the technical aspect of detecting a large proportion of skin-tone in a given image--though a close up of the back of someones hand would surely be flagged; but could anyone be offended by it?

The real problem comes in identifying what is likely to offend the whole gamut of possible offendees.

Take the recent story of the Aussie Banker seen on live TV with "nude pictures" of a model on his screen. The girl is a model and spends much of her career being photographed scantily clad. Do an image search of her and you'll find it nigh impossible to find a photo of her fully clothed. Religious prohibitions aside, is it really so shocking? Offensive?

A freind of mine has a large photo of her (then) early 20s daughter hanging on her living room wall. In the photo the daughter is entirely naked, and very heavily pregnant. Strategically placed arm and thigh prevent anything more than might show in a bikini being visible. The oblique lighting and a high contrast black and white print emphasis the motherly curves and make for a very beautiful and completely tastful photo. But I can easily imagine that the same persons offended by the model above would find it offensive.

She also had the same photo as a screen saver on her laptop. She's a very proud mother and grandmother. But your "skin tones" test would likely completely fail to pick it up.