不管是鼠标经过超链接还是经过层，都可以实现
javascript方式： 熟悉使用document.getElementById()取得节点对象
<!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><html xmlns="http://www.w3.org/1999/xhtml">
<style>
.div_n{
width:300px;
height:250px;
border:1px solid gray;
}
</style>
<script type="text/javascript">
function changeSrc1()
{
document.getElementById("myImage").src="/images/2.gif"
}
function changeSrc2()
{
document.getElementById("myImage").src="/images/1.gif"
}
</script>
</head>
<body>
<div class="div_n">
<a href="http://www.baidu.com">
<img id="myImage" src="/images/1.gif" onmouseover="changeSrc1()" onmouseout="changeSrc2()"/>
</a>
</div>
</body>
</html>
css+div方式： 合理控制层的样式
<!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><html xmlns="http://www.w3.org/1999/xhtml">
<title> New Document </title>
<style>
.main{
width:300px;
height:250px;
border:1px solid gray;
}
.content{
width:150px;
height:160px;
border:1px solid gray;
background-image:url(images/2.gif);
background-repeat: no-repeat;
}
.content:hover{
background-image:url(images/3.jpg);
}
a{
text-decoration:none;
}
</style>
</head>
<body>
<div class="main">
<a href="http://www.baidu.com/"><div class="content"></div></a>
</div>
</body>
</html>