Try this way
1)
<div id="divWait" style="display: none;">
<img src='<%=ResolveUrl("~/images/plsWaitImg.gif")%>' alt="Please wait while loading data" style="vertical-align: middle;" />
loading...<br />
<br />
Please wait...
</div>
Then write Javascript to hide this div
2) Using Ajax
<asp:UpdateProgress AssociatedUpdatePanelID="updatePnlID" ID="prgId1" runat="server">
<ProgressTemplate>
<img src='<%=ResolveUrl("~/images/plsWaitImg.gif")%>' alt="Please wait while loading data" />
Please wait...
</ProgressTemplate>
</asp:UpdateProgress>
<asp:UpdatePanel ID="upnlStats" runat="server" UpdateMode="Conditional">
............
</asp:UpdatePanel>
Good Luck !!