Yes you can edit galaxies. You can just google or go to Hubblesite or similar places and download a picture of the galaxy. Face on galaxies are the best, because those are the ones that it will work with. Simply go to your Media folder (C:\Program Files\Universe Sandbox\Media), and find the pictures called "Galaxy-X", and replace it with the new one you had. Make sure the file names are exactly the same, and you rename the original and keep it.
Try the left option menu, it will be down towards the very bottom (scroll).