photobomb

/F OW1 T OW0 B AH0 M/
noun
  1. 1

    An act of photobombing.

  2. 2

    A photo containing someone or something that is photobombing.

verb
  1. 1

    To unexpectedly appear in a photograph, especially so as to ruin the picture.

Source: https://en.wiktionary.org/wiki/photobomb