Have you tried creating another User account to see it the problem can be re-created?
With me only having Home Premium I don't have a lot of experience with the Group Policy Editor, but after reading up on your situation I ran across a similar problem.
You should be able to get to it with your Pro edition.
The solution was to:
Disassociate whatever policies you have with your desktop and/or wallpaper, and reboot your machine. I surmise this would be part of any of your user accounts.
Then set your wallpaper to center instead of stretch. If that setting is already picked, click off of it and then click back on it. Apply, Save Changes, and OK your way out.
Then re-apply the policy, reboot your machine to see if it corrected itself.. Local Group Policy Editor - Open