If your are using Update Panel, then you can use the following code.
<asp:ToolkitScriptManager ID="ToolkitScriptManager1" runat="server">
</asp:ToolkitScriptManager>
<asp:UpdatePanel ID="UpdatePanel1" runat="server">
<ContentTemplate>
Your data and Form
</ContentTemplate>
</asp:UpdatePanel>
//Drag and Drop the UpdateProgress Control
<asp:UpdateProgress ID="UpdateProgress1" runat="server">
<ProgressTemplate>
<img src="image_path.gif" />
</ProgressTemplate>
</asp:UpdateProgress>
//Add Extender
<asp:AlwaysVisibleControlExtender ID="UpdateProgress1_AlwaysVisibleControlExtender"
runat="server" Enabled="True" TargetControlID="UpdateProgress1" VerticalSide="Middle"
HorizontalSide="Center">
</asp:AlwaysVisibleControlExtender>