Display 'Loading' image while the gridview is getting populated

Question

I have few text fields in my c# page.When the user fills those fields and clicks on submit button, a Gridview is displayed as the result.

The problem here is that, the gridview will take long time to get fully populated since the query that runs behind scans large amounts of data.

So till the Gridview is getting popluated I was to display a Loading image so that users don't feel that system has crashed.I dont want to insert sleep statement like most of the examples shown on the net as that will increase my total time for output.How should I go about? Please Help!

   <form id="form1" runat="server">
     <div>
     <b>Enter value 1 :</b>
        <asp:TextBox id="p1" runat="server" /> <br />
     <b>Enter value2 :</b>
        <asp:TextBox id="p2" runat="server" /><br />
        <asp:Button ID="btn1" runat="server" OnClick="Button1_Click" Text="Start Search"/>
        <hr />

         <asp:ScriptManager ID="ScriptManager1" runat="server">
        </asp:ScriptManager>
        <asp:updatepanel id="ResultsUpdatePanel" runat="server">    
    <contenttemplate>

    <div style="text-align:center;">
    <asp:updateprogress id="UpdateProgress1" runat="server" associatedupdatepanelid="ResultsUpdatePanel" dynamiclayout="true">
                        <progresstemplate>

                           <img src="loader.gif">

                        </progresstemplate>
                    </asp:updateprogress>

                    </div>

         <asp:GridView ID="GridView1" runat="server" AutoGenerateColumns="true"
                    OnPageIndexChanging="gridView_PageIndexChanging"
                    CssClass="gridstyle" 
                    EnableViewState="false"
                    AllowPaging="true">
                    <AlternatingRowStyle CssClass="altrowstyle" />
                    <HeaderStyle CssClass="headerstyle" />
                    <RowStyle CssClass="rowstyle" />
                    <RowStyle Wrap="false" />
                    <HeaderStyle Wrap="false" />
                </asp:GridView>
    </contenttemplate>
</asp:updatepanel>
    </div>
    </form>