Is there a word for "photobomb" in Japanese?
In English, to photobomb someone is to spoil a photograph (of a person or thing) by unexpectedly appearing in the camera's field of view as the picture is taken, typically as a prank or practical joke.
映り込み is one option, but if you want a more catchy and specific word, there is a word 見切れ【みきれ】, which is a noun form of the verb 見切れる.
For example, Origami Cyclone is an anime character who is a serious "photobomber", and he is known as 見切れ職人 ("Artisan of 見切れ") in the Japanese version.
See this article for another example.