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
Interesting question from a theoretical / algorithmic standppoint. One approach to the problem would be to flag images that contain large skin-colored regions (as explained by Trull).
However, the amount of skin shown is not a determinant of an offesive image, it's rather the location of the skin shown. Perhaps you can use face detection (search for algorithms) to refine the results -- determine how large the skin regions are relative to the face, and if they belong to the face (perhaps how far below it they are).