新手ajax问题 - 编程论坛

// JScript 文件
var xmlHttp;
function creatHttp()
{
  if(window.ActiveXObject)
  {
     xmlHttp=new ActiveXOject("Microsoft.XMLHTTP");
     }
  if(window.XMLHttpRequest)
  {
  xmlHttp=new XMLHttpRequest();
  }

}
function checkName(name)
{
  creatHttp();
  var url="check.aspx?Name="+name;
  xmlHttp.open("GET",url,true);
  xmlHttp.onreadystatechange=backName;
  xmlHttp.send(null);
}
function backName()
{
if(xmlHttp.readyState==4)
{
   if(xmlHttp.status==200)
   {
     if(xmlHttp.responseText=="true")
     {document.getElementById("img1").src="image/1.jpg";}
     else
     {document.getElementById("img1").src="image/2.jpg";}
   }
}
}
//checkName.asp文件
using System.Collections;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;

public partial class check : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
        string name = Request.QueryString["Name"].ToString();
        if (name == "aaa")
        { Response.Write("true");
        Response.End();
    }
        else
        { Response.Write("false");
          Response.End();
    }

    }
}

//注册页面
<head runat="server">
    <title>无标题页</title>
    <script type="text/javascript" src="jss.js">
    </script>
</head>
<body>
    <form id="form1" runat="server">
    <div style="text-align: center">
        <aspabel ID="Label1" runat="server" Height="47px" Text="实现ajax技术" Width="164px"></aspabel>
        <table style="width: 274px; height: 123px">
            <tr>
                <td style="width: 100px">
                    用户名</td>
                <td style="width: 100px">
                    <input id="tsname" type="text" onkeyup="checkName(document.getElementById('tsname').value);" /></td>
            </tr>
            <tr>

当在用户名对话框中输入字符时图片不改变..
刚学入门望指教先谢谢了