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
Rigan Ap-apid presented a paper at WorldComp '08 on just this problem space. The paper is allegedly here, but the server was timing out for me. I attended the presentation of the paper and he covered comparable systems and their effectiveness as well as his own approach. You might contact him directly.