Google keeps your settings and restores them after you first sign in. You can prevent this by doing the following. When you first boot up and touch the Android to continue, choose to skip sign in. Go into Settings/Privacy and uncheck "Back Up my data" and "Automatic Restore". Then go ahead and log in. You won't see your old wallpaper. Of course then you won't have your other settings either, including the auto-sync of your apps. If you backed up your apps and settings using Titanium backup then its not a problem. Once you log back in, you can always re-check the boxes in your Privacy settings.