spoil a photograph of (a person or thing) by unexpectedly appearing in the camera's field of view as the picture is taken, typically as a prank or practical joke

we were interrupted and photobombed by at least twenty tourists

a photograph that has been spoiled by the unexpected appearance of an unintended subject in the camera's field of view as the picture was taken

everyone loves a good photobomb and it's even more hilarious when celebrities get in on the action