Convert featured images into “product images”

That sounds weird because post thumbnails were changed to Featured Images with WP 3.0.

I guess you’re talking about some custom field or metabox your framework adds on post edit. You could grab _thumbnail_id which is the key in the wp_postmeta table that stores what you’re looking for.

But to me you’d better usewp_get_attachment_img_src:

if(has_post_thumbnail($post->ID))
{
   $img = wp_get_attachment_img_src(get_post_thumbnail_id($post->ID), 'the_image_size');
   //  URL is the first data of the array
   echo $img[0];
}