photobomber
n.— «A “photobomber” is someone who sneaks into one of your happy pics and ruins it. You’re standing there with your new partner, smiling for the camera, capturing the happy moment….absolutely oblivious to the fact that some charlie without pants is mounting the horse statue behind you.» —“From the desk of the Traffic Guy” Big Breakfast Blog (South Africa) June 5, 2008. (source: Double-Tongued Dictionary)