the HTML for button creation is located in the html_output.php file in the includes/functions/ folder. It pulls some info from the tpl_product_display.php template page and together with the function creates the button. If you simply need to change the button image from .gif to say .png or .jpg that can be done easily in the includes/languages/english/button_names.php file where you find your DEFINES. You just change the button name from 'button_add_to_cart.gif' to 'button_add_to_cart.png'. That might be what your looking for.