photobombing

/F OW1 T OW0 B AA1 M IH0 NG/
verb
  1. 1

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

noun
  1. 1

    The practice or phenomenon of making an unexpected appearance in a photograph, especially with the deliberate intention of ruining it.

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