Photobomb is a comparatively new Internet slang term referring to someone jumping into a photo shoot without the photographer's permission. The original(?) definition of photobombing, as put forth by Scott Lamb, is as follows:
"Intentionally turning up in the background of other people’s photographs with the goal of ruining them now has a name: Photobombing. I do this daily, unintentionally, walking along Canal Street to work, but the art of the photobomber is appearing in the background at just the right moment and with just the right face."
This quote is the earliest reference to the term that I have been able to find, from a post at BuzzFeed on May 06, 2008. This was posted by Scott Lamb, who is an editor of BuzzFeed. Scott gave an interview on NPR on May 15, and at this time he talked about 'photobombing' as if it was an established term, but I suspect he was just just proud of the new buzzword that he had invented. Urbandictionary.com doesn't record the term until November 2, 2009. Google trends shows a sudden explosion of searches for 'photobomb' in May 2009, with no notable use before then. Google trends also gives us the interesting fact that photobomb is primarily a Canadian term, with Australians being the second most common to search for the term. America comes in third, with the UK coming in fourth.
Photobomb is currently an extremely popular term, use incessantly on sites like Digg, Reddit, and Thisisphotobomb (a sister site of Icanhascheezburger.com). It is often used to refer to any photograph in which something unexpected happened, although it is most correctly used to refer to a person appearing in a photo of which they are not an intended subject.