<style>
.docBg { width:100%; height:100%;
    position:fixed;
    left:0;
    top:0;
    z-index:-1;
    _position:absolute; /* IE6 */
    _top:expression(eval(document.documentElement.scrollTop)); /* IE6 */
}
</style>
       
<!-- ------------------------------------------------------- -->
       
<div class="docBg">
    <img class="bg" src="images/bg.jpg" style="width:100%; height: auto;" alt="页面背景"/>
</div>