When you write the css document, you have a problem in PNG background images that are transparent. You can put other attribute for ie 6 that if when is Ie6 exchange to another image:
Img { background-image:url(../images/img_DespInf_Selected.png);} _background-image:url(../images/img_DespInf_Selected.gif);}
The other way, is use css whit javascript. I see a javascript very small and is perfect to work whit css. The link is http://labs.unitinteractive.com/unitpngfix.php . Very small script (3kb). Put the script on the head tag and them put this code in html/php page
<!–[if lt IE 7]>
<script type=”text/javascript” src=”unitpngfix.js”></script>
<![endif]–>
Direct link to http://labs.unitinteractive.com/downloads/unitpngfix.zip
Antuane's Css 