sure is. Go to the Library folder at the root of yr HD, not your User folder. then go to Desktop Pictures. Find "Aqua Blue.jpg" and drag it out, store it some place safe for later if you want. Choose the new picture you want to use and name it "Aqua Blue.jpg" and put it in place of the old one. That's it. Just keep in mind this will appear on the Login screen for all users, not just yours.