issue with update panel and button click

I am trying to create a webpage where user will select items from 4 listboxes and then click a submit button and upon pressing a button three things will happen: 1) show a loading image while data is being fetched from server 2) when data fetching is completed, hide the loading image and display a table  3) display charts (code behind using c# and asp.net chart control). 
This is the code I have for loading image on button click in .aspx file.
var prm = Sys.WebForms.PageRequestManager.getInstance(); // Add initializeRequest and endRequest prm.add_initializeRequest(prm_InitializeRequest); prm.add_endRequest(prm_EndRequest); // Called when async postback begins function prm_InitializeRequest(sender, args) { //display loading image and set it to visible var panelProg = $get('divImage'); panelProg.style.display = ''; } // Called w ...