Sorry, a bit snowed under at the moment.

Basically, the 'magic' happens in function zen_image . In there there is a function 'handle_image' which acts as a hook for all the IH stuff. Or any other image manipulation you might want to do. So, that is where IH hooks into the existing system.

But :

below $products_image_large for the additional image in the loop is defined as
images/large/file_a.jpg
Don't understand what you are saying here. where is 'file_a.jpg' coming from?