photobomb
[ foh-toh-bom ]
/ ˈfoʊ toʊˌbɒm /
verb (used with object)
to ruin (a photograph) by appearing in the image without the photographer’s knowledge, often in some dramatic or comical way: My brother photobombed what would have been a nice photo of my grandma and me.
to do this to (the subject of a photograph or the photographer): I thought I had a great picture, but I got photobombed by a pigeon flying by.
noun
a photograph that is ruined in this way.