Useless Definition: (verb)- To drop in a photo unexpectedly. To hop in a picture right before it is taken.
sarah: hey why is jimmy in the background of my myspace picture?
ryan: idk, he must have photobombed it at the last second.
Another Useless Definition: Any time the background of a picture hijacks the original focus.
Uncle Bob's hairy ass totally photobombed that picture.