title: Please explain this code
body: <p>Trying to make sense of the <a href="https://gist.github.com/268257" rel="nofollow">code below</a>, as I want to customise it a bit.
  As per the comments, the <code>callback function is passed the last image to load as an argument</code>. Can someone please explain how the callback is passed <strong>only to the last image</strong> (I understand <em>why</em>, I just don't know <em>how</em>)? From the way I'm reading the code it seems to be passed to all of them.</p>
  
  <p>Cheers.</p>
  
  <pre class="lang-js prettyprint-override"><code>// $('img.photo',this).imagesLoaded(myFunction)
  // execute a callback when all images have loaded.
  // needed because .load() doesn't work on cached images
  
  // mit license. paul irish. 2010.
  // webkit fix from Oren Solomianik. thx!
  
  // callback function is passed the last image to load
  //   as an argument, and the collection as 'this'
  
  
  $.fn.imagesLoaded = function(callback){
    var elems = this.filter('img'),
        len   = elems.length,
        blank = 'data:image/gif;base64,' +
            'R0lGODlhAQABAIAAAAAAAP///ywAAAAAAQABAAACAUwAOw==';
  
    elems.bind('load',function(){
        if (--len &lt;= 0 &amp;&amp; this.src !== blank){ callback.call(elems,this); }
    }).each(function(){
       // cached images don't fire load sometimes, so we reset src.
       if (this.complete || this.complete === undefined){
          var src = this.src;
          // webkit hack from
          // groups.google.com/group/jquery-dev/browse_thread/thread/eee6ab7b2da50e1f
          // data uri bypasses webkit log warning (thx doug jones)
          this.src = blank;
          this.src = src;
       }  
    }); 
  
    return this;
  };
  </code></pre>