This is not really a question for this forum but more of a general "how to" on CSS/HTML.
Changing HTML elements from text to graphic is all done the same way. So for your particular case, you would want something like the following:
.woo_buy_button_container button.button {display:block;text-indent:-9999px;background:url(images/yourgraphic.png) no-repeat scroll 0 0 transparent;width:30px;height:70px;outline:none;border:none;}Of course this is just an example and you have to alter it to your site for it to work.