The Add to cart button is actually a gif file.
It is located at:
Replace YOUR_JOOMLA_FOLDER and YOUR_DEFAULT_TEMPLATE with the corresponding values for your web site installation.
Replace this gif file with one designed by you and reload the web site. The new Add to cart button will be visualized in your online shop.
You can also change the label of the button from the following file:
Replace USED_LANGUAGE with the corresponding language file name (for example english.php).
Look for the following line and enter the preferred value:
‘PHPSHOP_CART_ADD_TO’ => ‘Add to Cart’,