That's a good question. I asked a few people I know about its origin and they said the Hubble or the Shuttle took it. That would be wrong as the only way to get a pretty spiral picture of our galaxy like that is from outside it.
You know how the moon is sometimes a crescent, and sometimes a circle? It's because of this very phenomenon. The angle the sun hits the moon changes and you can see more or less shadow depending on what part of the cycle the moon is in.