The change the add to cart button you edit add_to_cart.tpl.
change:
{include file="buttons/button.tpl" button_title=$lng.lbl_add_to_cart href=$href title=$title style=$style}
to:
{include file="buttons/atcButton.tpl" button_title=$lng.lbl_add_to_cart href=$href title=$title style=$style}
Ok so atcButton.tpl does not yet exist, so you open button.tpl with your text editor and save a copy of it as atcButton.tpl
NOTE: make sure you don't save over button.tpl.
Make the appropriate changes to you new file and upload the add_to_cart.tpl and the new atcButton.tpl to the buttons folder and you've got it.
you can check out
www.affordableimrpint.com to see it in action. I have changed all of the buttons. You can also make them change when hovered over.