From Urban Dictionary:
A photo bomber is someone who either intentionally or unintentionally ruins an otherwise normal photo.
The “photo bomber” will be doing such things as: making faces, gestures, naked, or getting naked, in a costume, or doing some other equally hilarious action.
The photo bomber will usually work in the background of the photo. Some of the more ballsy photo bombers will go as far as being within inches behind or even next to the people in the picture , or in rare cases, run in FRONT of the photo.
The most ideal way to photo bomb is to successfully ruin their photowithout letting the people know that you are forever ruining what could have been an amazing photo and memory, except you’re sitting on a bench 15 yards behind them while you purposely have your cock and balls hanging out the top of your shorts while you eat an ice cream cone seductively.