How To Lock your Desktop Wallpaper?

One of my friend was strugging for this, asking to all that how to lock my desktop wallpaper..because his friends every time change that wallpaper.
there are simple steps:
>goto 'run'. 
>type 'gpedit.msc' 
>goto 'user configuration' 
>goto 'adminstrative policy 
>goto 'desktop' 
>goto active desktop
there is option active desktop wallpaper

Do enable that and enjoy :)