To change the current desktop wallpaper and make this change effective immediately, PowerShell can tap into the windows API calls. Here is a function that changes the wallpaper immediately:
function Set-Wallpaper { param( [Parameter(Mandatory=$true)] $Path, [ValidateSet('Center', 'Stretch')] $Style = 'Stretch' ) Add-Type @" using System; using System.Runtime.InteropServices; using Microsoft.Win32; namespace Wallpaper { public enum Style : int { Center, Stretch } 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; } key.Close(); } } } "@ [Wallpaper.Setter]::SetWallpaper( $Path, $Style ) } Set-Wallpaper -Path 'C:\Windows\Web\Wallpaper\Characters\img24.jpg'