protected void jugar_Click(object sender, ImageClickEventArgs e)
{
    String turno = (string)Session["Turno"];
    ImageButton boton = (ImageButton)sender;
    if (turno == "1")
    {
        boton.ImageUrl = "C:UsersEpyrosdocumentsvisual studio 2015ProjectsEnTA TE TIEnTA TE TIImagenesTic-tac-toe-cross.png";
        Session["Turno"] = 2;
    }
    else if (turno == "2")
    {
        boton.ImageUrl = "C:UsersEpyrosdocumentsvisual studio 2015ProjectsEnTA TE TIEnTA TE TIImagenesTic-tac-toe-nought-e1461667111145.png";
        Session["Turno"] = 1;
    }
}