$W32Wallpaper = @'
using System.Runtime.InteropServices;
namespace Win32{
    public class Wallpaper{
        [DllImport("user32.dll", CharSet=CharSet.Auto)]
        static extern int SystemParametersInfo (int uAction , int uParam , string lpvParam , int fuWinIni);
        public static void SetWallpaper(string thePath){
            SystemParametersInfo(20,0,thePath,3);
        }
    }
}
'@
Add-Type $W32Wallpaper

[Win32.Wallpaper]::SetWallpaper([string]::Empty)