Поставь это в CSS:

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+*/
}

ИЛИ ТАК

img:hover {
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+*/
}