When performing an Ajax request I want to change the submit button into a loading image and then swop it back again when the Ajax request is complete.

I don't want a loading image anywhere else as on mobile phones it can't always be seen due to small screen size.

I have grabbed the event (which references the form, is that right?) and I have something like this:

Code:
<button type="submit" name="name_can_vary">Search</button>
event.name_can_vary.style.visibility = 'hidden';
//(obviously this isn't complete code but no doubt explains what I am talking about)
..which would hide the button and works exactly as I want to this point, but of course it does;t return when Ajax has completed or show the loading image.

Ideally, instead of just hiding the button I want JavaScript to swop the button for the loading image. I suppose I could have extra HTML on every page that is made visible with the loading image but I would rather have JS do everything from here (saves editing all pages with submit buttons on).

Thanks for any help.