photobombing

Photobombing:

To move within a frame right before it’s taken, therefore ruining the shot for the original posers, or appear unexpectedly (like a bomb) in a photograph. So if you crane your neck to put yourself in the picture or make a face in the background, you’re photobombing. Check this out: http://www.huffingtonpost.com/2013/01/21/malia-obama-photobomb_n_2522543.html

And this is a classic example (see Bill Clinton’s face):
http://www.capitalfm.com/artists/kelly-clarkson/news/bill-clinton-photo-bomb/