Photobomb

‘Photobomb’ is when an unexpected person or thing that either intentionally or by accident appears as the photograph is being taken. Photobombing usually happens as a practical joke, and without the acknowledgement of the main subjects of the photo.
Example of use: “I can’t believe that almost all of our wedding photos were photobombed by that crazy pigeon. In one of them you can even see the moment he tried to attack my husband.”