Set-Wallpaper

Function Set-WallPaper($Value) {
	Set-ItemProperty -path 'HKCU:\Control Panel\Desktop\' -name wallpaper -value $value
	rundll32.exe user32.dll, UpdatePerUserSystemParameters
}

.NET

$SetWallpaperSrc = @'
using System.Runtime.InteropServices;
public class wallpaper {
    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 ) {
        SystemParametersInfo( SetDesktopWallpaper, 0, path, UpdateIniFile | SendWinIniChange );
    }
}
'@
Add-Type -TypeDefinition $SetWallpaperSrc
 
[wallpaper]::SetWallpaper($PathToSaveImagesAndSavedImageName)