cosi funziona:
codice:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" >
<head>
<style type="text/css">
a.opacity img {
	filter:alpha(opacity=20); 
	-moz-opacity: 0.2; 
	opacity: 0.2;
	border:0px;
} 
a.opacity:hover img {
	filter:alpha(opacity=100); 
	-moz-opacity: 1.0; 
	opacity: 1.0;
	border:0px;
}
.OpacityOnTxt img {
	filter:alpha(opacity=10);
	-moz-opacity: 0.1; 
	opacity: 0.1;
	border:0px;
}
</style>

</head>
<body>
<div id="Due">[img]images/bariera_negativo.png[/img]</div>
alfa
</body>
</html>
Inserendo l'id nel div e impostando opacità meno di 100 (anche perché 100[visibilità piena] 0 [Invisibilità totale])