var AlphaChanneledPNG = Class.create();
AlphaChanneledPNG.prototype = {
  initialize: function(imgId, blankImgSrc) {
    this.imgId = imgId;
    this.blankImgSrc = blankImgSrc;
    Event.observe(window, 'load', this.show.bind(this));
  },
  show: function() {
    if (/MSIE (5\.5|6\.)/.test(navigator.userAgent)) {
      imgObj = $(this.imgId);
      orgSrc = imgObj.src;
      imgObj.src = this.blankImgSrc;
      imgObj.style.filter = "progid:DXImageTransform.Microsoft.AlphaImageLoader("
                          + "src='" + orgSrc + "', sizingMethod='scale')";
    }
  }
};

var foo = new AlphaChanneledPNG('foo', 'images/blank.gif');