Ajax Loader Part 2

I have server software running for templates. When the user has finished editing the template they click a button that takes them to a proof page. The button uses javascript and what I need is for a loader gif to appear while the server is thinking about loading the next page as some of the templates are quite large and can take a few seconds before it does anything and I need to let the user know something is happening. . The page also uses prototype-1.6.0.3 and scriptaculos