How to Show loading Image between each page postback.

You can use Simple AJAXControl for that. Try to Explore UpdatePanel and provide the UpdateProject Image for the same.

<asp:ScriptManager ID="ScriptManager1" runat="server" />
       <asp:UpdateProgress runat="server" id="UpdateProgress">
           <ProgressTemplate>
               Loading Please Wait... // Here you can put a loading image as well
           </ProgressTemplate>
       </asp:UpdateProgress>
       <asp:UpdatePanel runat="server" id="Panel">
           <ContentTemplate>
Different ASP.NET Content that will participate in Partial Page rendering 
               <asp:Button runat="server" id="buttonSave" onclick="Save_Click" text="Save" />
           </ContentTemplate>
       </asp:UpdatePanel>

Hope this will help.