Someone visible in a photograph but who was not meant to be in it - i.e. a picture of a group of friends has a random person walking past in the background with a funny expression. Origins - Fantasy
Football (
mid 1990's TV programme) feature about how Nigel Clough would
always try to sneak into the camera shot. Usually used in the context 'doing a Nigel'. Kudos is always given to 'Nigels' with funny facial expressions or particularly if you can deliberately 'Nigel' someone else's photo.