I believe I may have blogged about Tineye before. That's an image-analyzer website that combs the 'net and can determine what other images were used as source-material for your picture. My other photoshop buddy and I have been constantly trying to beat Tineye: using screen-capture tools, doing colorizing and pixel modifications...at each attempt, Tineye was able to see right through us.
Only recently, have I discovered a way to obfuscate the eye-of-tin.
You simply mirror your image.