上传图片显示

<asp:Content ID="Content1" ContentPlaceHolderID="head" runat="server">
<!--<script language="javascript">
  function dop()
  {
    __doPostBack('lbtnupload','');
  }
</script>-->
</asp:Content>
<asp:Content ID="Content2" ContentPlaceHolderID="ContentPlaceHolder1" runat="server">
<table cellpadding="0" cellspacing="0" border="0" width="100%">
   <tr>
     <td>
        <asp:Image ID="pic" runat="server" />
        <asp:FileUpload ID="pic_upload" runat="server" onchange="javascript:__doPostBack('lbtnupload','')" />
        <asp:Label ID="lbl_pic" runat="server"></asp:Label>
        <asp:LinkButton ID="lbtnupload" runat="server" onclick="lbtnupload_Click"></asp:LinkButton>
     </td>
  </tr>
</table>
</asp:Content>
我用母板页做的   onchange="javascript:__doPostBack('lbtnupload','')  这里点击上传后  利用javascript 自动获取<asp:LinkButton ID="lbtnupload" runat="server" onclick="lbtnupload_Click">中的onclick="lbtnupload_Click" 事件     但是  用母板页做  却不能执行onclick="lbtnupload_Click"  这个事件？请问 我应该怎么做才能点击   FileUpload图片上传后自己显示在 Image上