How to assign background image of button to a variable using jquery css

How to assign background image of button to a variable using jquery css

Hi,
I want to assign background image of button in a jquery variable with css.I have written code but it not work.

var cmp1;
var cmp2;

var bg = $("#mq3").css('background-image');

    var bg1 = $("#mq6").css('background-image');

    var bg2 = $("#mq9").css('background-image');


$(document).ready(function() {

$("#mq9").click(function() {

        //     alert("Hello Welcome");
        if ($("#mq9").css("background-image") === bg2) {
            $("#mq9").css("background-image", "url('images/bkv.png')");


            cmp1 = $("#mq9").css('background-image');
            cmp2 = $("#mq3").css('background-image');
            cmp3 = $("#mq6").css('background-image');
           
            if (cmp1 === cmp3) {
                alert("welcome mohit");
                //setTimeout("window.open('Qtwo.html')",3000);
            }
        } else {
            $("#mq9").css("background-image", "url('images/cmst.png')");
            bg2 = $("#mq9").css('background-image');
        }

    });

});