Add to cart button missing

[snorkerz]

Using the andover modern template, I have this line in my button_names.php folder

define ('BUTTON_IMAGE_ADD_TO_CART', 'button_add_to_cart.gif');

I have an image 'button_add_to_cart.gif' in includes/templates/andover_modern/buttons/english

but on the site all that is generated is the text based Alternative button, which I have set in button_names.php to read 'order now'.

How can I get my gif button to show instead?

Do I have a choice of size?

Example at http://tenancyservices.snorkerz.com/...products_id=14