Hi,
Had a chat with Ed and the solution would be to check if the function exists, if it does create the dynamic image sizes, else send the larges image.
Open content-product.php
and locate this:
$image = sf_aq_resize( $thumb_img_url, $thumb_width, $thumb_height, true, false );
Replace with this and re-test:
if (function_exists('sf_aq_resize')) {
$image = sf_aq_resize( $thumb_img_url, $thumb_width, $thumb_height, true, false );
} else {
$image = $thumb_img_url;
}