$("#pic_list ").on("click","img", function() {

        var big_img_src = this.src.replace("small/","");
        load_big_pic(big_img_src);

    });

    //图片等比自适应并居中
    function load_big_pic(big_img_src) {

        $('#big_img').attr('src',big_img_src);
        var img = new Image();
        img.src = big_img_src;

        img.onload = function(){
            var imgWidth ;
            var imgHeight ;

            imgWidth = this.width;
            imgHeight = this.height;

            if(imgWidth<1200 && imgHeight<400)
            {
                $("#big_img").width(imgWidth);
                $("#big_img").height(imgHeight);
            }
            else
            {
                newWidth = 400*imgWidth/imgHeight;
                $("#big_img").width(newWidth);
                $("#big_img").height(400);
                //让图片居中显示
                var margin = (1200 - newWidth) / 2;
                $("#big_img").css("margin-left", margin);
            }
        };

    }