Hola,
Mira el siguiente ejemplo que anexo, no es mio, pero sirve a tus propositos..
/*
Gradual-Highlight Image Script-
© Dynamic Drive (
www.dynamicdrive.com)
For full source code, installation instructions,
100's more DHTML scripts, and Terms Of
Use, visit dynamicdrive.com
*/
function high(which2){
theobject=which2;
highlighting=setInterval("highlightit(theobject)", 50);
}
function low(which2){
clearInterval(highlighting);
which2.filters.alpha.opacity=20;
}
function highlightit(cur2){
if (cur2.filters.alpha.opacity<100)
cur2.filters.alpha.opacity+=5;
else if (window.highlighting)
clearInterval(highlighting);
}
</script>
</head>
<body>
<a href="whatever.htm"><img alt="" src="c:/caraya/Nuvaring/images/top.jpg" border="0"
style="filter:alpha(opacity=40)" onMouseover="high(this)"
onMouseout="low(this)"></a>
</body>
saludos.