Close button image in jQuery dialog does not appear

I created a modal window, the window is working, the button in the right corner appears, the event to close the window works perfectly, the image on the button does not appear. What should I do?

function MessageBox(titulo, mensagem) {

    $('body').append('<div id="janelaMensagem" title="' + titulo + '" style="display:none"> <p>' + mensagem + '</p> </div>');

    $(function () {
        $("#janelaMensagem").dialog({
            modal: true,
            close: function () {
                $(this).remove();
            },
            buttons: {
                "Fechar": function () {
                    $(this).dialog("close");
                    $(this).remove();
                }

            }
        });
    });

}