Especially in cities, where sharing photos with the people within 100 feet of you can include anyone in your 20 story apartment building. Even if the rate of obscene photos is low, just one could ruin the user experience.

What does constitute to you as obscene and why would it ruin your user experience?

Dicks, mostly.

Penises, because I don't want to see anyone else's.

Good point. I think it has to tie into your social network. Maybe you can adjust the degree of freedom -- more depth = more risk.

But the concept that people could do nude pictures and then be around the corner to actually see your reaction in real life is kind of creepy.