@ ScriptJunkie (and others)

OK, I did some digging, and I dont think the problem has anything to do with the additional_images file.
I made a scratch installation of ZC1.3.9 and IH2. out of the box (w/o ZLB) I was seeing the exact same behavior you described (the main product image was being called from /images and NOT bmz_cache)

I started digging thru the files and I found that after 2 tries of merging the includes directory, [includes/modules/(theme)/main_product_image.php] was not getting updated to the IH2 version.

after manually installing this 1 file, I stared seeing expected behavior of the main product image.

after this, I installed ZLB 1.6.2 I was seeing the proper behavior still. (the lightbox image was pulled from bmz_cache and NOT /images)

So check your server and see if that file was updated to the IH2 version.

Daniel