The infamous spinner as done by JQuery:

Create a div that contains the image that you want to use as a "loader" image.

<div id="loader">
  <img alt="" src="path/to_graphic" />
</div>

It just takes a small piece of Javascript (JQuery) to make the magic happen

$('#loader').hide() //this initially hides the div
   .ajaxStart(function(){
     $(this).show(); // show the div when the Ajax call starts
   )}
   .ajaxStop(function(){
     $(this).hide(); //hide the div when the Ajax call stops
   )};

Place the loader div within the div that's receiving the results of the ajax call, and JQuery code with the rest of your JQuery code and you have a quick and dirty loader