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);
   -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+*/
}