If you are having the problem where the additional image thumbnail shows on the product info page but when you click to see it on slimbox you just see loading spiral constantly and no image because the link to the image goes to the images/large/ folder to look for it but doesn't exist then I've found a solution.
*** Edit the original additional_images.php file that comes with Fual Slimbox ***
Basically open up includes/modules/templates/YOUR_TEMPLATE/additional_images.php
and remove the "large/" on line #81
#81 $products_image_large = ereg_replace('^' . DIR_WS_IMAGES, DIR_WS_IMAGES . 'large/', $products_image_directory) . ereg_replace($file_extension . '$', '', $file) . IMAGE_SUFFIX_LARGE . $file_extension;
This fixed my problem as described above so I hope this helps some of you guys out!
PS. This is an awesome script buddy great job!