AJAX developers need to be able to tell their users when the browser is waiting for a response from a network call. Usually the text message is accompanied an animated .GIF like these ones:
On http://www.ajaxload.info/ you can make your own animated GIF icon - there's tons of different templates to choose from and you can, of course, set the fore- and background colors (or even make it transparent if that's what you need). Also take a look at http://www.napyfab.com/ajax-indicators/ for even more indicators.