This fixes a background Image users cannot chance.
To use this function navigate along :

- HKEY_CURRENT_USER\Software\Microsoft
\CurrentVersion\Policies\System

If System does not exist you nee to create it in the left pane.
Rightclick Policies , new , subkey and name it System.

In the right pane under System create 2 REG_SZ [String Value] entries
and name them as below:
Wallpaper
WallpaperStyle

Doubleclick the Wallpaper entry and set it's value to the path of the Image
File you want to have as background.
Now doubleclick WallpaperStyle and define the value to:
0 = Centered(default) , 1 = Tiled , 2 = Stretched

Note:I testet WallpaperStyle and it worked without a restart.