hi.
here a simple class to change the wallpaper in winxp and win7. i think it will work on every windows version.

public sealed  class clsWallpaper
        {
            
            const int SPI_SETDESKWALLPAPER = 20;
            const int SPIF_UPDATEINIFILE = 0x01;
            const int SPIF_SENDWININICHANGE = 0x02;

            [DllImport("user32.dll", CharSet = CharSet.Auto)]
            static extern int SystemParametersInfo(int uAction, int uParam, string lpvParam, int fuWinIni);

            public enum Style : int
            {
                Tiled,
                Centered,
                Stretched
            }

            public static void Set(Uri uri, Style style)
            {
                System.IO.Stream s = new System.Net.WebClient().OpenRead(uri.ToString());

                System.Drawing.Image img = System.Drawing.Image.FromStream(s);
                string tempPath = Path.Combine(Path.GetTempPath(), "wallpaper.bmp");
                img.Save(tempPath, System.Drawing.Imaging.ImageFormat.Bmp);

                RegistryKey key = Registry.CurrentUser.OpenSubKey(@"Control Panel\Desktop", true);
                if (style == Style.Stretched)
                {
                    key.SetValue(@"WallpaperStyle", 2.ToString());
                    key.SetValue(@"TileWallpaper", 0.ToString());
                }

                if (style == Style.Centered)
                {
                    key.SetValue(@"WallpaperStyle", 1.ToString());
                    key.SetValue(@"TileWallpaper", 0.ToString());
                }

                if (style == Style.Tiled)
                {
                    key.SetValue(@"WallpaperStyle", 1.ToString());
                    key.SetValue(@"TileWallpaper", 1.ToString());
                }

                SystemParametersInfo(SPI_SETDESKWALLPAPER,
                    0,
                    tempPath,
                    SPIF_UPDATEINIFILE | SPIF_SENDWININICHANGE);
            }
        }
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