There are a lot of fixes for Alpha PNG transparancy in Internet Explorer 6.  They all have their pros and cons, but I love this one because (unlike some other fixes) it covers every image at once.  That way, you can put in a lot of transparent images and get a massive fix for all.

<!--[if gte IE 5.5000]>
<script language="JavaScript">
function correctPNG()
{
   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
      }
   }
}
window.attachEvent("onload", correctPNG);
</script>
<![endif]-->

The only downside: any transparent parts of the image will not be clickable for hyperlink purposes.  If you have a white logo on a transparent background, for example, you can only click on the white parts, and not the image as a whole.