Warning: Serious problems might occur if you modify the registry incorrectly by using Registry Editor or by using another method. These problems might require that you reinstall the operating system. Microsoft cannot guarantee that these problems can be solved. Modify the registry at your own risk. See How to back up the registry

Step by Step:
1. Open regedit (aka registry editor; you can access it from the star menu by typing “regedit” in the search box…)
2. Go to “HKEY_CURRENT_USER\Control Panel\Desktop\” folder and click on it
3. On the right hand side find there are a bunch of entries. Find the key named “wallpaper” and double click on it and put the path of the picture you want as your new wallpaper (example path is “C:\Users\Bob\Pictures\new_wallpaper.jpg”)
4. Right-click on the “Desktop” folder in regedit that you found in step 2 and click Permissions.
5. Click “Advanced”
6. Go to “Owner” tab, highlight your name in the box that says ‘Change owner to’ ( There are only two choices the other is Administrator )… once your user name is highlighted click “OK”
7. Click on “Advanced” again
8. Uncheck the button that reads “Include inheritable permissions from the object’s parent”… click “Remove” when prompted
9. Click “Add”
10. Type “Everyone” and click “OK”
11. Check Allow “Read Control” and click “OK”
12. Click “OK” again
13. Highlight “Everyone” and check to Allow “Read” and click “OK”
14. Restart Computer and enjoy new Wallpaper