要么用js，或htc插件，要么用ie滤镜。。以下提供滤镜解决方案
/* for ie6 */
*html .big_title {
     background:none;
     filter:progid:DXImageTransform.Microsoft.AlphaImageLoader(src='images/qiye2.png', sizingMethod='crop');}
ps:滤镜只能用作背景图片，直接用<img>标签引用的话滤镜解决不了

用png.js文件来过滤的代码：

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);
 // JavaScript Document

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