I’ve had this one for a little bit now, but I’m just getting around to hunting down the source. After a bit of creative Googling I’ve managed to find out this picture comes from the Smoke on the Water article at Wikipedia.
The caption reads:
The sign in a guitar store on Denmark Street, London: “Absolutely, under no circumstances at all are customers allowed to play Stairway to Heaven, Sweet Child of Mine, or Smoke on the Water while trying out guitars. Thank You.”