I read an article the other day where a woman said she checked photos of guys before she went on a date. She said she often found that the guy on a dating site was also in Facebook where he had a wife and two children. Very clever for her.
I wondered how she did that, so I tried out several image search tool. I took a photo from my website, changed the file name, and then looked for copies of it. Bing Image Search and Tineye both found copies of the photo, where other people have, well, borrowed the photo from my website. The best tool was Google Image Search, which found dozens of copies of my photo. (Update: Yandex added image search at yandex.com/images/search.)
To use this, click Google Image Search and drag your photo onto the page.