$(".main-slider").bxSlider({
            //mode:'fade',
            controls:false,
            pager: 'true',
            auto:false,
            onSlideBefore:function($slideElement, oldIndex, newIndex){
                var newBg = $slideElement.find("span.banner-bg  img").attr("src");
                $slideElement.css( 'background-image', 'url(' + newBg+ ')');
            },
            onSliderLoad: function(currentIndex){
                $(".main-slider li").each(function(){
                        var newBg = $(this).find("span.banner-bg  img").attr("src");
                        $(this).css( 'background-image', 'url(' + newBg+ ')');
                });

            }
        });