photobombingpho‧to‧bomb‧ing /ˈfəʊtəʊˌbɒmɪŋ $ ˈfoʊtoʊˌbɑː-/ noun [uncountable] 
the activity of deliberately getting into the background of someone else’s photograph and spoiling it by doing something such as making a silly expression with your face
the activity of deliberately getting into the background of someone else’s photograph and spoiling it by doing something such as making a silly expression with your face