条件：事先需要做成图片
原理：随机调取图片
案例：
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"
"http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<title>随机生成登录图片验证码</title>
</head>
<body>
<table>
<tr>
<div id='div1'></div>
</td>
</tr>
</table>
</body>
<script language="javascript">
var str = "";
var img = "";
var strsource = ['是', '天', '日', '科', '的', '会', '更', '好', '创', '新'];
for(var i = 0; i < 4; i++) {
var n = Math.floor(Math.random() * strsource.length);
str = str + strsource[n];
img = img + "<img src='Images/checkcode/" + n + ".gif' width='19' height='20'> "; //图片路径
div1.innerHTML = img; //设置div1的输出
}
</script>
</html>运行：
