WebClient client = new WebClient();
client.OpenReadCompleted += (sender, event) => {
	using (IsolatedStorageFile storage = IsolatedStorageFile.GetUserStoreForApplication())
    {
		using (Stream tileStream = storage.CreateFile("Shared/ShellContent/tile.png"))
        {
			evt.Result.CopyTo(tileStream);
        }
    }
 
	ShellTile tile = ShellTile.ActiveTiles.First();
    tile.Update(new StandardTileData
    {
		BackgroundImage = new Uri("isostore:/Shared/ShellContent/tile.png")
    });
};
client.OpenReadAsync(new Uri("http://forum.aspitalia.com/avatars/90/Ricciolo.avatar"));