For our little demonstration, we're going to use a page hit counter from blogskinny.com, an image from ajaxload.info, and some CSS;
background:transparent url(images/ajaxload.gif) center center no-repeat;
And to make the background image appear as we want it, we need to add the width and height properties of the image, with a bit of inline-css styles in a div surrounding the image.
It's a bit difficult to see, so try a Ctrl+F5 refresh and concentrate on the image!
It's a simple idea, and one I'll be using in my upcoming redesign of the site...