codice:
// WBchat 2.0a [08/09/2006]
// Simone Cingano (imente)
// http://www.imente.it/wbchat

/* FUNCTION PNGFIX
 * 		correctly handle PNG transparency in Win IE 5.5 or higher
 *
 * THANKS TO 	http://homepage.ntlworld.com/bobosola. Updated 02-March-2004
 * MODIFIED BY 	imente
 */

function pngfix() {
	
	for(var i=0; i<document.images.length; i++)
	  {
	  var img = document.images[i]
	  var imgName = img.src.toUpperCase()
	  if (imgName.substring(imgName.length-3, imgName.length) == "PNG")
		 {
		 var imgID = (img.id) ? "id='" + img.id + "' " : ""
		 var imgClass = (img.className) ? "class='" + img.className + "' " : ""
		 var imgTitle = (img.title) ? "title='" + img.title + "' " : "title='" + img.alt + "' "
		 var imgStyle = "display:inline-block;" + img.style.cssText 
		 if (img.align == "left") imgStyle = "float:left;" + imgStyle
		 if (img.align == "right") imgStyle = "float:right;" + imgStyle
		 if (img.parentElement.href) imgStyle = "cursor:hand;" + imgStyle		
		 var strNewHTML = "<span " + imgID + imgClass + imgTitle
		 + " style=\"" + "width:" + img.width + "px; height:" + img.height + "px;" + imgStyle + ";"
		 + "filter:progid:DXImageTransform.Microsoft.AlphaImageLoader"
		 + "(src=\'" + img.src + "\', sizingMethod='scale');\"></span>" 
		 img.outerHTML = strNewHTML
		 i = i-1
		 }
	  }
	  
}

if (is_ie5_5up && !is_ie7_7up) { window.attachEvent("onload", pngfix); }
L'autore e' giustamente citato...
ciao