我用timer和updatepanel做的一个刷新img图片，会闪屏 请问怎么解决

请问可以用其它方法防止闪屏吗？
代码如下：
<%@ Page Language="C#" AutoEventWireup="true" CodeFile="Default2.aspx.cs" Inherits="Default2" %><!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 runat="server">
    <title></title>
</head>
<body>
    <form id="form1" runat="server">
<% =DateTime.Now.ToString()%> 
    <asp:ScriptManager ID="ScriptManager1" runat="server">
        </asp:ScriptManager>
        <asp:UpdatePanel ID="UpdatePanel1" runat="server">
            <ContentTemplate>
                <div><% =DateTime.Now.ToString()%> 
       放大X轴 <asp:DropDownList ID="DropDownList1" runat="server" AutoPostBack="true" 
            onselectedindexchanged="DropDownList1_SelectedIndexChanged">
            <asp:ListItem>1</asp:ListItem>
             <asp:ListItem>2</asp:ListItem>
              <asp:ListItem>3</asp:ListItem>
               <asp:ListItem>4</asp:ListItem>
                <asp:ListItem>5</asp:ListItem>
        </asp:DropDownList>
    </div>
             <div><img src=""; alt="" id="img" runat="server" /></div>
                <asp:Timer ID="Timer1" runat="server" Interval="5000" ontick="Timer1_Tick">
                </asp:Timer>
            </ContentTemplate>
        </asp:UpdatePanel>
    
   
    </form>
</body>
</html>cs.
  protected void Page_Load(object sender, EventArgs e)
    {
      if(!IsPostBack)  img.Src = "Default.aspx?id=1";
    }
    protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)
    {
       string s = DropDownList1.SelectedValue;
       img.Src = "Default.aspx?id="+s;
    }
    protected void Timer1_Tick(object sender, EventArgs e)
    {
        string s = DropDownList1.SelectedValue;
        img.Src = "Default.aspx?id=" + s + "&time=" + DateTime.Now.ToLongTimeString() ;
       
    }