PowerShell: Change desktop wallpaper on remote computer

Hey,

I was trying to have some fun with my boyfriend, chaning his wallpaper. I couldn't figure it out, so I asked him for help, but he couldn't figure it out either. We both run win7, he's running PowerShell v2 and I got v3. We added each-others computers to trusted hosts, set execution policy to unrestricted, enabled psremoting, configured a home group, and know each-others credentials (admin). This is what we have tried:

Add-Type @"
using System;
using System.Runtime.InteropServices;
using Microsoft.Win32;
namespace Wallpaper
{
   public enum Style : int
   {
       Tile, Center, Stretch, NoChange
   }
   public class Setter {
      public const int SetDesktopWallpaper = 20;
      public const int UpdateIniFile = 0x01;
      public const int SendWinIniChange = 0x02;
      [DllImport("user32.dll", SetLastError = true, CharSet = CharSet.Auto)]
      private static extern int SystemParametersInfo (int uAction, int uParam, string lpvParam, int fuWinIni);
      public static void SetWallpaper ( string path, Wallpaper.Style style ) {
         SystemParametersInfo( SetDesktopWallpaper, 0, path, UpdateIniFile | SendWinIniChange );
         RegistryKey key = Registry.CurrentUser.OpenSubKey("Control Panel\\Desktop", true);
         switch( style )
         {
            case Style.Stretch :
               key.SetValue(@"WallpaperStyle", "2") ; 
               key.SetValue(@"TileWallpaper", "0") ;
               break;
            case Style.Center :
               key.SetValue(@"WallpaperStyle", "1") ; 
               key.SetValue(@"TileWallpaper", "0") ; 
               break;
            case Style.Tile :
               key.SetValue(@"WallpaperStyle", "1") ; 
               key.SetValue(@"TileWallpaper", "1") ;
               break;
            case Style.NoChange :
               break;
         }
         key.Close();
      }
   }
}"@

[Wallpaper.Setter]::SetWallpaper( 'C:\temp\Wallpaper.bmp', 2 )

invoke-command { powershell.exe -noprofile -executionpolicy Bypass C:\temp\ChangeDesktop.ps1 } -computername Lucius-PC -Credential $cred

Invoke-Command -ComputerName Lucius-Pc { Set-ItemProperty -Path "HKCU:Control Panel\Desktop" -name WallPaper -value c:\temp\wallpaper.bmp } -cred $cred