Intentionally posing in other people's photos, for a later surprise.
Usually people making funny faces in the background, without the knowledge of the main subjects of the photo.
Jim is always photobombing, he messed ups some of the best photos from the wedding.
Photophobia is a fear of, or adverse physical or psychological reaction to light. Whether from rapid change of light intensity, or sudden burst of bright light, as from a camera flash. Not to be confused with heliophobia, which is a fear of the Sun.
When we went to the laser light show, Jon must've got a case of photophobia. He puked and had a seizure.