IE 6 does not fully support transparency, or the alpha channel that png’s allow. There are many hacks and tricks to achieve this, but the best I have come across yet is to quarantine your IE fixes within a conditional statement, like such:
<!-- ************** png fix for browsers older than ie 7 ************** -->
<!--[if lt IE 7]>
<link rel="stylesheet" type="text/css" media="screen" href="css/ie-png.css" />
<![endif]-->
<!-- ******************************************************************* -->
Then for the css file (in this case “ie-png.css”)
/* IE7 does not support the star-html hack anymore, so this is ignored by IE7: */
* html #yourElement{
background: transparent none;
filter:progid:DXImageTransform.Microsoft.AlphaImageLoader(src='img/logo.png',sizingMethod='scale');
}