Вот что-то типа такого…
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>Untitled Document</title>
<script type="text/javascript" src="jquery-1.4.js"></script>
<script type="text/javascript">
$(document).ready(function() {
$("#img img:eq(1)").hide();
$("#img").hover(
function() { $(this).find("img").animate({ opacity: "toggle" }, "slow"); },
function() { $(this).find("img").animate({ opacity: "toggle" }, "slow"); }
);
});
</script>
</head>
<body>
<div style="position:relative;" id="img">
<img src="img1.png" style="position:absolute; left:0; top:0;" />
<img src="img2.png" style="position:absolute; left:0; top:0;" />
</div>
</body>
</html>