const int SPIF_UPDATEINIFILE = 0x01;
const int SPIF_SENDWININICHANGE = 0x02;
const int SPI_SETDESKWALLPAPER = 0x14;

public enum TileWallpaper
{
    None = 0,
    Tile = 1
}

public enum WallpaperStyle
{
    Center = 0,
    Tile = 1,
    Stretch = 2,
    Fit = 6,
    Fill = 10,
    Span = 22
}

public static void SetWallpaper(string filePath, WallpaperStyle wallpaperStyle)
{
    using (RegistryKey registryKey = Registry.CurrentUser.OpenSubKey(@"Control PanelDesktop", true))
    {
        TileWallpaper tileBit;
        if (wallpaperStyle == WallpaperStyle.Tile)
        {
            tileBit = TileWallpaper.Tile;
        }
        else
        {
            tileBit = TileWallpaper.None;
        }
        registryKey.SetValue(@"TileWallpaper", ((int)tileBit).ToString());
        registryKey.SetValue(@"WallpaperStyle", ((int)wallpaperStyle).ToString());

        int returnCode = Win32.SystemParametersInfo(SPI_SETDESKWALLPAPER,
            0,
            filePath,
            SPIF_UPDATEINIFILE | SPIF_SENDWININICHANGE);
    }
}