TinEye is a great service that you can use to search for similar photos on the web. You provide a photo and it compares it to its database looking for similar and modified images.
You can use TinEye to quickly spot fake accounts on social networking sites.
For example. I received this LinkedIn network request the other day.
Not only have I never worked with a “Jennifer Gray”, her profile photo looks like it may be a stock photo. TinEye returned 4 results for stock photography.
Looks like this account may be a recruiting bot or something.
TinEye can also be used to verify the authenticity of a photo and to see if it is a repost or duplicate of another photo. It even has Firefox and Chrome plugins!