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.

The fine art of ruining other people's photos. Usually by running in the background or making a silly face in the background. It's usually done to strangers, but shit man, you better run if they notice at the moment, because you might get your ass kicked.

The act where one or several persons ruin (sometimes improve) a photo by performing funny acts in the background which may include a dry gangbang, holding stick like objects up to your crotch or raising your clothing.

The fine art of ruining other peoples photos

Benedict Cumberbatch at the oscars.

Any picture involving someone with an awkward smile, or an open mouthed attempt.