Solution for a common problem: "Cannot write to the registry"
RegistryKey reg = Registry.CurrentUser.OpenSubKey("Control Panel\\Desktop"); reg.SetValue("WallpaperStyle", "1"); //2 for stretch
Modified (working):
RegistryKey reg = Registry.CurrentUser.OpenSubKey("Control Panel\\Desktop", true); reg.SetValue("WallpaperStyle", "1"); //2 for stretch