在<head></head>之间加上如下代码：
<Script Language="JavaScript">
image = new Array(4); //定义image为图片数量的数组
image [0] = 'tu0.gif' //背景图象的路径
image [1] = 'tu1.gif'
image [2] = 'tu2.gif'
image [3] = 'tu3.gif'
image [4] = 'tu4.gif'
number = Math.floor(Math.random() * image.length);
document.write("<BODY background="/+image[number]+">");
</Script>