鼠标经过背景图片变换

<style type="text/css">
.a{background-image:url(images/2008.jpg)}
.b{background-image:url(images/2009.jpg)}
</style>
<div style="width:200px;height:200px;" class="a" onmousemove="this.className='b'" onmouseout="this.className='a'"></div>