请教 背景图片怎么随显示器大小而变啊

#Layer1 {
position:absolute;
width:479px;
height:115px;
z-index:1;
left: 300px;
top: 550px;
}body {
background-image: url(images/5.jpg);

}</style>
</head>
<body >
<div id="Layer1">
  <form runat="server">
  <table border="0">
  <tr>
   <td> 
         <asp:ImageButton ID="Ibcx" runat="server" ImageUrl="images/dd.gif" 
             onclick="ImageButton1_Click" />
   </td>
   <td>
        <asp:ImageButton ID="Ibbm" runat="server" ImageUrl="images/e.gif" 
            onclick="Ibbm_Click" />
   </td>
   <td>
        <asp:ImageButton ID="Ibtc" runat="server" ImageUrl="images/r.gif" 
            onclick="Ibtc_Click" />
   </td>
 </tr>
 </table>
 </form>
</div>
</body>