<div id="bj"><img src="img/xx.jpg" /></div>
<p>多写点文字测试...</p>
...
<p>多写点文字测试...</p>

     body{
        background-image:url(../img/xx.jpg);
        background-position:center; 
        background-repeat:no-repeat; 
          }
      div#bj{
        position:fixed;
        top:0;
        left:0;
        bottom:0;
        right:0;
        z-index:-1;
          }
      div#bj > img{
        height:100%;
        width:100%;
        border:0;
          }