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.
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[ 3.3 ms ] story [ 16.6 ms ] threadIt works only because Twitter uses a white background for the thumbnail view, and a black background for the full view.