求大神给改下代码 自动弹出一个含图片的层 且背景变灰

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"  "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html>
<head>
<script type="text/javascript" src="jquery-1.5.1.min.js"></script>
<style type="text/css">
    .mydiv
    {
        text-align: center;font-size: 9pt;z-index: 99;width:500px;height:300px;left: 50%;top: 50%;
        margin-left: -250px !important;margin-top: -150px !important;margin-top: 0px;position: fixed !important;
        position: absolute;
        _top: expression(eval(document.compatMode && 
            document.compatMode=='CSS1Compat') ? 
            documentElement.scrollTop + (document.documentElement.clientHeight-this.offsetHeight)/2 :/*IE6*/ 
            document.body.scrollTop + (document.body.clientHeight - this.clientHeight)/2); /*IE5 IE5.5*/
    }
    .SContent-box
    {
        width:500px;
        height:300px;
        background:red;
    }
    .bg
    {
        background-color: #666;
        width: 100%;
        height: 100%;
        left: 0;
        top: 0;
        filter: alpha(opacity=50);
        opacity: 0.5;
        z-index: 1;
        position: fixed !important; /*FF IE7*/
        position: absolute;
        _top: expression(eval(document.compatMode && document.compatMode=='CSS1Compat') ? 
            documentElement.scrollTop + (document.documentElement.clientHeight-this.offsetHeight)/2 :/*IE6*/ 
            document.body.scrollTop + (document.body.clientHeight - this.clientHeight)/2); /*IE5 IE5.5*/
    }
</style>
<script language="javascript" type="text/javascript">
    var Itemindex = 0;
    function showDiv() {
        document.getElementById('popDiv').style.display = 'BLOCK';
        document.getElementById('bg').style.display = 'block';       
    }
    function closeDiv() {
        document.getElementById('popDiv').style.display = 'none';
        document.getElementById('bg').style.display = 'none';
    }
</script>
</head>
<body><div onclick="showDiv()" style="display:block; cursor:pointer">
    点击弹出div
</div>
<div id="popDiv" class="mydiv" style="display: none;">    <div class="SContent-box">
        <div class="Close_btn">
             <a href="#" onclick="closeDiv()"  style="display:block; cursor:pointer">点击关闭</a>
        </div>
        
    </div>
</div>
<div id="bg" class="bg" style="display: none;">
</div></body>
</html>
<script>
window.onload=function(){showDiv()}
</script>