Thank you for your answer. I'm confused as to which button name I'm looking for. How do I find which button image the code is searching for?

To answer your suggestion: I show-- define('BUTTON_IMAGE_CONFIRM_ORDER_GREEN', 'btn_submit_order_green.gif'); in buttons_names.php and
an image named btn_submit_order_green in buttons/english

As you can see I'm very lost..