js 单个图片的延迟加载

<div id="banner">
    <img src="" trueImg="1.png"/>
</div>

 var banner = document.getElementById("banner");
    var imgFir = banner.getElementsByTagName("img")[0];

    window.setInterval(function(){
        var oImg = new Image;
        oImg.src = imgFir.getAttribute("trueImg");
        oImg.onload = function(){
            imgFir.src = this.src;
            imgFir.style.display = "block";
            oImg = null;
        }
    },500);