'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."
Photobombing has been around since the 70's, however, it became a widespread phenomenon following the technological transition from analog film to digital in the early 2000s, coupled with the rise of photo-sharing social sites such as Facebook, Instagram and more. The first recorded use of the phrase was in the online dictionary of slang words and phrases, 'Urban Dictionary', on May 6th 2008.