I found out about TinEye a few minutes ago. It's a nice little search engine which allows you to find exact copies of a photo or even similar versions of it. You can either upload an image for comparison with their database, or point TinEye to a URL of the image you want to check.
The site is also used in Wikipedia for detecting the original source of uploaded images.
I know there is some other website which also allows similarity search of images, but I just can't remember the URL. Please leave a comment if you know of other such sites, thanks!