$('#modalWindow').on('click', '#send', function () {
        $.ajax({
            type: "POST",
            url: "/Home/LoadImage",
            data: $('form').serialize(),
            success: function (data) {
                alert('Привет');
            },
            error: function (error) {
                alert(error.responseText);
            }
        });
    });

[HttpPost]
        public ActionResult LoadImage(Car car)
        {}

$('#modalWindow').on('click', '#send', function () {
        $.ajax({
            type: "POST",
            url: "/Home/LoadImage",
            data: {
                "car": $('form').serialize(),
                "base64img": $('#autoPicture').attr('src')
            },
            success: function (data) {
                alert('Привет');
            },
            error: function (error) {
                alert(error.responseText);
            }
        });
    });

[HttpPost]
        public ActionResult LoadImage(Car car, string base64img)
        {}