checkbox和图片button问题

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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
    <title> new document </title>
    <meta name="generator" content="editplus" />
    <meta name="author" content="Gao YiXiang" />
    <meta name="email" content="[email protected]" />
    <meta name="keywords" content="javascript dhtml dom" />
    <meta name="description" content="I love web development." />
</head>
<body>
    <input type="checkbox" id="ck" />
    <input type="image" id="img1" src="1.gif" alt="1.gif" />
    <script type="text/javascript">
    <!--
document.getElementById("ck").onclick = function()
{
    var img = document.getElementById("img1");
    if (this.checked)
    {
        img.src = "2.gif";
        img.alt = "2.gif";
    }
    else
    {
        img.src = "1.gif";
        img.alt = "1.gif";
    }
};
    //-->
    </script>
</body>
</html>