创建一个函数，点击页面的左下角的按钮可以更换body的背景图片，可是一直无法实现，大神给看看

<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<meta http-equiv="X-UA-Compatible" content="IE=Edge">
<meta name="viewport" content="width=device-width, initial-scale=1">
<title>全球搜索,有问必应 (Bing)</title>
<style type="text/css">
*{margin: 0;padding: 0;font-family: "微软雅黑";list-style:none;}
a:link{color: lightgrey;}
a:hover{color: white;}
body{background: url(senlin.jpg) fixed;background-position:center center;overflow：hidden}
.nav{background-color:#444444;width:100%;height:45px; }
.leftNav{margin-left:30px;}
.leftNav a{text-decoration: none;font-size:13px;float: left;width:60px;height:45px;line-height:45px;list-style:none;text-align:center;}
.teshu a{width:90px;float: left;font-size: 12px;height: 45px;}
.logo{background: url(logo.png);width:107px;height: 53px;float: left;margin:120px  0  0 100px;}
.shuru{border:0;height:50px;line-height:50px;width:450px;font-color: black;float: left;margin:120px  0  0 15px;font-size:20px;}
.subMit{border:0;height:50px;line-height:50px;width:55px;font-color: black;float: left;background-image:url(search-button.jpg);margin:120px 0 0 0;}
.button1{width:50px;height:50px;float: right;margin-right: 200px;margin-top: 500px;}
</style>
<body id="main">
    <div>
        <div>
            <ul>
                <li><a href="#">图片</a></li>
                <li><a href="#">视频</a></li>
                <li><a href="#">学术</a></li>
                <li><a href="#">词典</a></li>
                <li><a href="#">地图</a></li>
                <li><a href="#">更多</a></li>
                <li><a href="#">|</a></li>
            </ul>
            <ul>
                <li><a href="#">OfficeOnline</a></li>
                <li><a href="#">Outlook.com</a></li>
            </ul>
        
        </div>
        
        <div>
        
        </div>
    </div>
    <div>    
    </div>
    <form>
        <input type="text"></input>
        <input type="submit" value=""></input>
    </form>
    <div>
        <img  src="button1.png" onclick="changeImage()" >
    </div>
<!--下面是准备创建的函数-->
<script>
    function changeImage()
    {
    var n = Math.round((Math.random()*4));
    var imgChange = document.getElementById("main");
    switch (n)
    {
        case 1:
        imgChange.style.background = url("animals.jpg");
        
        case 2:
        imgChange.style.background = url("cayuyuan.jpg");
        break;
        case 3:
        imgChange.style.background = url("lake.jpg");
        break;
        case 4:
        imgChange.style.background = url("gui.jpg");
        break;
        default:
    }    
}
</script>

</body>
慕粉3398407
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