把 images/card.jpg 换成你要保存的那个img标签的src属性值就行了。
<html>
<head>
<script language="javascript">
function saveit(src)
{
I1.document.location=src;
savepic();
}
function savepic()
{
if(I1.document.readyState=="complete")
I1.document.execCommand("saveas");
else
window.setTimeout("savepic()",10);
}
</script>
</head>
<body>
<input type="button" value="保存图片" name="B1" onclick="javascript:saveit('images/card.jpg');">
<iframe name="I1" style="display:none"></iframe>
</body>
</html>