- First you copy the wallpaper files to destination machines via GPP.
2) administrative template you have to mention the local user profile wallpaper path.
check client machine reg settings:
3) This problem occurs if the Wallpaper registry value in HKEY_CURRENT_USER\Control Panel\Desktop is either missing or empty.
Here are steps to make a registry modification.
1. Open the Group Policy Management Console . Right-click the Group Policy object (GPO) that should contain the new preference item, and then click Edit .
2. In the console tree under Computer Configuration or User Configuration , expand the Preferences folder, and then expand 3. the Windows Settings folder.
4. Right-click the Registry node, point to New , and select Registry Item .
5. In the New Registry Item dialog box, select an Action for Group Policy to perform. Here is a screenshot.
6. In the Value data box, type a value, such as c:\test\a.jpg, and then click OK.
Here is a link about configure a Registry Item.
Configure a Registry Item: