This is a classic categorical problem for machine learning. I'm surprised so many suggestions have involved formulating some sort of clever algorithm like skin

This is a classic categorical problem for machine learning. I'm surprised so many suggestions have involved formulating some sort of clever algorithm like skin detection, colors, etc. You could certainly use one of those for a baseline, but I'd bet machine learning would out-score most human-derived algorithms. Take a look at the scores for classifying dogs vs cats with 97% accuracy http://www.kaggle.com/c/dogs-vs-cats/leaderboard http://www.kaggle.com/c/dogs-vs-cats/leaderboard. You could use a technique of digitizing the image pixels and feeding to a learning algorithm, similar to http://www.primaryobjects.com/CMS/Article154.aspx http://www.primaryobjects.com/CMS/Article154.aspx.