HTML：

<img src="blank.gif" class="lazy" src="foo.png" width="1500" height="1800" alt="bar">  
             ▲                ▲  
             ║                ╚═══ The class will be used for the lazy loader below  
             ║  
             ╚═══ Use faulty gif here to hide it from showing before loaded

Hint: If you want the placeholder rectangle to be visible and in one color,consider using a 1×1 px image for blank.gif. It will load
very fast and will stretch nicely to your proportions,filling it with
the color of your choosing.

JavaScript的：

/* lazyload.js (c) Lorenzo Giuliani
 * MIT License (http://www.opensource.org/licenses/mit-license.html)
 *
 * expects a list of:  
 * `<img src="blank.gif" src="my_image.png" width="600" height="400" class="lazy">`
 */

!function(window){
  var $q = function(q,res){
        if (document.querySelectorAll) {
          res = document.querySelectorAll(q);
        } else {
          var d=document,a=d.styleSheets[0] || d.createStyleSheet();
          a.addRule(q,'f:b');
          for(var l=d.all,b=0,c=[],f=l.length;b<f;b++)
            l[b].currentStyle.f && c.push(l[b]);

          a.removeRule(0);
          res = c;
        }
        return res;
      },addEventListener = function(evt,fn){
        window.addEventListener
          ? this.addEventListener(evt,fn,false)
          : (window.attachEvent)
            ? this.attachEvent('on' + evt,fn)
            : this['on' + evt] = fn;
      },_has = function(obj,key) {
        return Object.prototype.hasOwnProperty.call(obj,key);
      }
    ;

  function loadImage (el,fn) {
    var img = new Image(),src = el.getAttribute('data-src');
    img.onload = function() {
      if (!! el.parent)
        el.parent.replaceChild(img,el)
      else
        el.src = src;

      fn? fn() : null;
    }
    img.src = src;
  }

  function elementInViewport(el) {
    var rect = el.getBoundingClientRect()

    return (
       rect.top    >= 0
    && rect.left   >= 0
    && rect.top <= (window.innerHeight || document.documentElement.clientHeight)
    )
  }

    var images = new Array(),query = $q('img.lazy'),processScroll = function(){
          for (var i = 0; i < images.length; i++) {
            if (elementInViewport(images[i])) {
              loadImage(images[i],function () {
                images.splice(i,i);
              });
            }
          };
        }
      ;
    // Array.prototype.slice.call is not callable under our lovely IE8 
    for (var i = 0; i < query.length; i++) {
      images.push(query[i]);
    };

    processScroll();
    addEventListener('scroll',processScroll);

}(this);

资料来源：
Lazyload脚本可以找到here.