I have managed to get this to work on 1.3.8p3.
Looking at the instructions I think English ( or even American English) is not the first language of the developer.
This is not his fault but it does make the instructions a bit more tricky :)
Before adding anything I changed a couple of directory names
I changed /admin/ to /ADMIN_OVERRIDE_NAME/ - ie what I have for my admin directory
and includes/modules/custom_template to includes/modules/TEMPLATE_OVERRIDE_NAME/ ie what I have for my template name
Also in attrib_prod_info.php I changed line 23 from
<?php echo '<a href="javascript:popupWindow(\'' . zen_href_link(FILENAME_POPUP_IMAGE_ADDITIONAL, 'products_image_large_additional=' . 'images/'.$attributes_image) . '\')">' . $image . '<br /><span class="imgLink">' . zen_image('images/bigger_picture.jpg', 'larger image', '140', '44') . '</span></a>'; ?>
to
<?php echo '<a href="javascript:popupWindow(\'' . zen_href_link(FILENAME_POPUP_IMAGE_ADDITIONAL, 'products_image_large_additional=' . 'images/'.$attributes_image) . '\')">' . $image . '<br /><span class="imgLink">' . TEXT_CLICK_TO_ENLARGE . '</span></a>'; ?>
ie removed the ref to images/bigger_picture.jpg which doesn't exist and so was getting a 404
I ftp'd up the files and applied the sql patch - I must admit I was a bit concerned that I had to add a file at the root of the zen cart install (attrib_prod_info.php) as this does not seem the correct place for it - however since I need the functionality I have put up with it for now
I then went into the attributes controller and added large images for the attributes - I could not point them to the path I wanted as it would only allow me 1 level of nesting - that is probably just a zen cart thing though
The end result is it works - though I can't say it works well out of the box -
To improve it I would wish better integration with Image Handler 2 and better use of overrides. - just my 2p worth
To end I would like to say I am a zen cart newbie so take what I say with a pinch of salt - as always before trying something you are unsure of back up your database and your site before applying any changes
Cheers
T