SetWallpaper.ps1

Set-ExecutionPolicy -ExecutionPolicy RemoteSigned -Scope CurrentUser -Force

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();
      }
   }
}
"@

# 0: Tile 1: Center 2: Stretch 3: No Change
[Wallpaper.Setter]::SetWallpaper( 'c:\temp\raspi-wallpaper.jpg', 2 )