I had to open this one up in GIMP to figure it out, but it's a .PNG with very cleverly applied alpha-channel transparency and shading. With a white background, you see the room, and with a black background, you see outer space.
It works only because Twitter uses a white background for the thumbnail view, and a black background for the full view.
It works only because Twitter uses a white background for the thumbnail view, and a black background for the full view.