The background image was cropped from a picture of the Orion Nebula, released in the public domain by NASA, ESA, M. Robberto (Space Telescope Science Institute/ESA) and the Hubble Space Telescope Orion Treasury Project Team.
I believe it is awesome that such pictures are made available in such an open fashion. Check out the original (324 MP!) in the Wikimedia Commons or check out http://hubblesite.org/
The assets for planets and fleets were sponsored by Alexander Holz. He provided them under the conditions of a CC-BY-SA license.