I kept that in mind, just make that one specific switch and it will leave your custom image in place.
Alternately you can just delete the button you have now, and take the entire code we provide for an add to cart button and replace only the image URL, http://www.e-junkie.com/ej/ejadd_to_cart.gif, with the URL of your custom image, that leaves all the other necessary parts in place.
In this case, either action will have the same result considering what you already have on your page.