a:hover img {     
              filter: progid: DXImageTransform.Microsoft.Alpha(opacity=100);     
              -moz-opacity: 1; /* Mozilla 1.6 */     
              -khtml-opacity: 1; /* Konqueror 3.1, Safari 1.1 */     
              opacity: 1; /* CSS3 - Mozilla 1.7b +, Firefox 0.9 +, Safari 1.2+, Opera 9 */     
              filter: alpha(opacity=100); /* IE 5.5+*/     
}     

a img {     
              filter: progid: DXImageTransform.Microsoft.Alpha(opacity=50);     
вставляйте в любое место CSS     
взято отсюда     
              -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+*/     
}