怎样实现鼠标移动到图象上面的时候换图片文件
代码:
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=gb2312">
<title>gouwo.com</title>
<script language="vb" runat="server">
Sub on_a(Source As Object, e As EventArgs)
img_oldbook.Src="picture/oldbook1.gif"
End Sub
</script>
</head>
<img id="a" src="picture/OldBook.gif" onMouseOver="on_a">
</html>
打开网页的时候提示错误:错误: 'on_a' 未定义
救命啊。。。。