Send image with ajax

Question

I am trying to receive a PHP image sent through AJAX, but I was not successful. Follow the code:

jQuery:

$("#envmsg").submit(function() {
    $("#btn-enviarmsg").button("loading");
    $("body").css("cursor", "wait");

    var dados = $(this).serialize();

    $.ajax({
        type: "POST",
        url: "/enviamsg.php",
        data: dados,
        success: function(data) {
            document.getElementById("envmsg").reset();
            $("#btn-enviarmsg").button("reset");
            $("body").css("cursor", "default");
            $("#deixarMensagem").modal("hide");
            location.reload();
        }
    });

    return false;
});

and in the enviar.php. i do the checking:

if($_FILES["foto"]) {
        $foto = echo "<script>alert('tem foto');</script>";
    } else {
        $foto = echo "<script>alert('n tem foto');</script>";
    }

But nothing occore, why?

Note: This form sends some text fields and the image. I just posted snippets of the code related to the problem.

Note: My form is enctype="Multipart/form-data".

Thank you!