JavaScript 网页背景图片随机化

 单调的网站首页背景看多了不免有写厌烦，如果用户自己每刷新一次页面背景可以更换也许能为这种单调添一丝的新奇。这只需用一个JavaScript方法就可以搞定，通过随机函数在我们的一组图片中随机的选一张作为背景。
下面是JavaScript代码：
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<mce:script type="text/javascript"><!--  
     // Add a class to the body tag to alternate background features  
     var class_options = new Array( "variation1", "variation2", "variation3" );  
 
     if (Math.random) {  
     var choice = Math.floor(Math.random() * (class_options.length));  
 
     // Just in case javascript gets carried away...  
     choice = ( (choice < class_options.length) && choice >= 0) ? choice : 0;  
 
     if (document.body.className == '') {  
     document.body.className = class_options[choice];  
     } else {  
     document.body.className += ' '+class_options[choice];  
     }  
     }  
-></mce:script> 
    <mce:script type="text/javascript"><!--
         // Add a class to the body tag to alternate background features
         var class_options = new Array( "variation1", "variation2", "variation3" );
         if (Math.random) {
         var choice = Math.floor(Math.random() * (class_options.length));
         // Just in case javascript gets carried away...
       &nb