Photobombing

[Merna]

Intentionally [posing] in [other peoples] photos, for a later surprise.
Usually people making funny faces in the background, without [the knowledge] of the main subjects of the photo.

[Julian]

The action of spoiling a photograph by unexpectedly appearing in the cameras field of view as the picture is taken, typically as a prank or practical joke.

[Sarah]

the act of appearing behind or in front of someone when their photograph is being taken, usually doing something silly as a joke: