I'd spend hour after hour searching for that perfect image for a campaign. You cook up this visual in your head of the perfect image, and then it is almost impossible to find an existing image that meets your imaginary criteria.
I recently heard from a contact at Masterfile about their new search functionality. It has been around for a few months now. It is AWESOME. It was actually honored at the Webby Awards. They have tools called "SimSearch" and "Visual Matching" where you can take an existing photo, and it will recommend other images that suit it well. I don't know how they do it. Tiny elves? Magic wands? Some kind of potion? (see why I don't work in tech?) However they did it, they did it well. Check out these examples:
Masterfile has posted a demo video here.