Prova così


codice:
<!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>Documento senza titolo</title>
<style type="text/css">
<!--
a {
	background: url(img1.jpg);
}
#l1 {
	height: 40px;
	width: 120px;
	display: block;
}
#l1:hover {
	background: url(img2.jpg);
	height: 40px;
	width: 120px;
	display: block;
}
-->
</style>
</head>

<body>

</body>
</html>