I do this in reverse quite frequently. I'm on OKCupid and when I want to find out more about someone I'm interested in, I use Google Images reverse image search (http://www.google.com/imghp) - click on the little camera and dump the img url. I would say about 30% of the time it leads me to their same photo on some other site, after which I have their full name and can do more background checking.
I assume you're aware of TinEye (http://www.tineye.com/), which sometimes gives different results. Google is generally better, but can't hurt to double-check.
