allora ti posto la prova che ho fatto io (copia e incolla)
funziona perfettamente ovunque

codice:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" lang="it" xml:lang="it">
    <head>
        <meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
        <title>xhtml 1.0</title>
     
        <style type="text/css">
        
        .highlightit img{
            filter: alpha(opacity=75);
            -moz-opacity = .75;
            opacity: .75;
        }

        .highlightit:hover img{
            filter: alpha(opacity=100);
            -moz-opacity = 1;
            opacity: 1;
        }
        
        a img { border: 0;}
        </style>

    </head>

<body>

    

.png</p>
    

    
    

.gif</p>
    

    
    

.jpg</p>
    


</body>
</html>