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+*/  
}