source: github/skins/default/pngbehavior.htc @ 20a1b3a

<public:component>
<public:attach event="onpropertychange" onevent="propertyChanged()" />
<script>

var supported = /MSIE (5\.5)|[6789]/.test(navigator.userAgent) && navigator.platform == "Win32";
var realSrc;
var blankSrc = "skins/default/images/blank.gif";

if (supported) fixImage();

function propertyChanged() {
   if (!supported) return;

   var pName = event.propertyName;
   if (pName != "src") return;
   // if not set to blank
   if ( ! new RegExp(blankSrc).test(src))
      fixImage();
};

function fixImage() {
   // get src
   var src = element.src;

   // check for real change
   if (src == realSrc) {
      element.src = blankSrc;
      return;
   }

   if ( ! new RegExp(blankSrc).test(src)) {
      // backup old src
      realSrc = src;
      element._original_src = realSrc;
   }

   // test for png
   if ( /\.png$/.test( realSrc.toLowerCase() ) ) {
      // set blank image
      element.src = blankSrc;
      // set filter
      element.runtimeStyle.filter = "progid:DXImageTransform.Microsoft.AlphaImageLoader(src='" +
                                     src + "',sizingMethod='scale')";
   }
   else {
      // remove filter
      element.runtimeStyle.filter = "";
   }
}

</script>
</public:component>