Sorry mate, I think the bottom line here is you aren't going to get around manual checking. There are approaches, but none really reliable and with a lot of false positives.
Check out this question: What is the best way to programatically detect porn images? for inspiration.
If you ask me though, it's a waste of time to look for an automated solution. Given half an hour's time, I can find you twenty images that would trigger a "nude" alarm even though perfectly innocent, and the same the other way around. Also, nudity is not going to be the only thing you do not want on your site.
Better spend the time on a system that makes it really easy to manually verify the content users upload to your site, e.g. as a Desktop widget, on the mobile phone, or whatever suits you best.
