To restrict users from changing the wallpaper (under your Administrator account):
- Click on Start > Run > type gpedit.msc and press enter
- Go to Local Computer Policy > User Configuration > Administrative Templates > Desktop
- In the right pane, select Desktop wallpaper and enable it
- Indicate the full path for your custom/default wallpaper
- Center and stretch the image as required and press on the OK button to validate.
- Restart your PC to check the results.