转自Dynamic Drive，纯CSS的，原理就是在a标记里面放两张图片，一张大图，一张小图，正常情况下小图可见，大图隐藏，a:hover就显示大图了
以下是引用片段：
<style type="text/css">
.thumbnail{
position: relative;
z-index: 0;
}
.thumbnail:hover{
background-color: transparent;
z-index: 50;
}
.thumbnail span{ /*CSS for enlarged image*/
position: absolute;
background-color: lightyellow;
padding: 5px;
left: -1000px;
border: 1px dashed gray;
visibility: hidden;
color: black;
text-decoration: none;
}
.thumbnail span img{ /*CSS for enlarged image*/
border-width: 0;
padding: 2px;
}
.thumbnail:hover span{ /*CSS for enlarged image on hover*/
visibility: visible;
top: 0;
left: 60px; /*position where enlarged image should offset horizontally */
}
</style>
<a class="thumbnail" href="#thumb"><img src="http://www.dynamicdrive.com/cssexamples/media/tree_thumb.jpg" width="100px" height="66px" border="0" /><span><img src="http://www.dynamicdrive.com/cssexamples/media/tree.jpg" /><br />Simply beautiful.</span></a>
<a class="thumbnail" href="#thumb"><img src="http://www.dynamicdrive.com/cssexamples/media/ocean_thumb.jpg" width="100px" height="66px" border="0" /><span><img src="http://www.dynamicdrive.com/cssexamples/media/ocean.jpg" /><br />So real, it's unreal. Or is it?</span></a>
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