I am building an ASP.NET web site where the users may upload photos of themselves. There could be thousands of photos uploaded every day. One thing my boss has asked a few time
I would approach the problem from a statistical standpoint. Get a bunch of pictures that you consider safe, and a bunch that you don't (that will make for a fun day of research), and see what they have in common. Analyze them all for color range and saturation to see if you can pick out characteristics that all of the naughty photos, and few of the safe ones have.