how to put a message while waiting for callback from ajax

I would like to insert an image working.gif while the data is being processed by the PHP post.. right now it just does nothing until data is returned.. sometimes there is a 10-15 second processing time before data is returned so some sort of indication to tell the user to wait will be great. Lets use the working.gif image as that indicator.

Appreciate assistance on how i can factor the above in to the following code:

$.ajax({
    type: "POST",
    url: "post.php",
    data: dataString,

    //if received a response from the server
    success: function (response) {

        $('#ajaxResponse').html('<pre>' + response + '</pre>');