<div class="wrap">

    <div style="background: url(images/by_01.jpg) no-repeat top center;"></div>

    <div style="background: url(images/by_02.jpg) no-repeat top center;"></div>

    <div style="background: url(images/by_03.jpg) no-repeat top center;"></div>

</div>

$(".wrap div").each(function(){

    var img=$(this);

    var url=$(this).css('backgroundImage');

    //console.log(url);

    s = url.match(/url\((.*?)\)/);

    url =s[1];

    if(url[0]=="\""){url = url.slice(1,-1)}

    //url=url.substring(5,url.length - 2);

    //console.log(url);

    $("<img/>").attr("src", url).load(function() {

        realWidth = this.width;

        realHeight = this.height;

        //如果真实的宽度大于浏览器的宽度就按照100%显示

        img.css("height",realHeight+"px");

    });

            /*

            var img = $(this);

            var realWidth;//真实的宽度

            var realHeight;//真实的高度

            //这里做下说明，$("<img/>")这里是创建一个临时的img标签，类似js创建一个new Image()对象！

            $("<img/>").attr("src", 'images/'+$(img).attr("data-image")).load(function() {

                realWidth = this.width;

                realHeight = this.height;

                //如果真实的宽度大于浏览器的宽度就按照100%显示

                if(realWidth>=_w){

                    $(img).css("height",realHeight+"px");

                }

                else{//如果小于浏览器的宽度按照原尺寸显示

                    $(img).css("width",realWidth+'px').css("height",realHeight+'px');

                }

            });*/

});