Pictures, usually movie, used in a film as a background to commentary. Implies that they would not have been included if better footage was available.
Something happens which has been filmed for 20 seconds, but to explain it takes 40 seconds. Pictures of the expert explaining it are boring, so 10 seconds of "wallpaper" are needed (e.g. exterior of factory.) Also known as padding.
When describing something or someone, if they are 'wallpaper' they not the art on the walls, they are more like the wall paper. You would not get interesting wallpaper, because you cant change it if you get sick of it, so you get something neutral, simple, uneventful.
I liked their first album, but the rest have been just wallpaper.