a:hover img {  
   filter: progid: DXImageTransform.Microsoft.Alpha(opacity=50);  
   -moz-opacity: 0.5; /* Mozilla 1.6 */  
   -khtml-opacity: 0.5; /* Konqueror 3.1, Safari 1.1 */  
   opacity: 0.5; /* CSS3 - Mozilla 1.7b +, Firefox 0.9 +, Safari 1.2+, Opera 9 */  
   filter: alpha(opacity=50); /* IE 5.5+*/  
}