<style type="text/css">

.list{position:relative;}

.list span img{ border-width: 0;padding: 2px; width:200px;}

.list span{position: absolute;padding: 2px;border: 1px solid #ccc;visibility: hidden;background-color:#FFFFFF;}

.list:hover{background-color:transparent;}

.list:hover span{visibility: visible;top:-150px; left:35px;}

</style>

<a class="list" href="#"><img src="/img/small.jpg" /><span><img src="/img/big.jpg" alt="大图" /></span>