Add attachments to lightbox

Thanks for your compliment and quick support!
The first two links point to an url. That’s normal and that’s what i wanted. I use a custom field to determine if it’s a “link” or a “lightbox”
The problem is on the third link “Buyer’s guide – Salon Première Classe”
i have attachmed images on this post and i need my lightbox to display the attached images and not only one.
in fact i have to open a group of images and not only one.
Hope my english is good enough :)

I think that is what i have.
I have a post with 3 images (associated to a featured thumbnail) but when i click on the image that only open one image and not the others. maybe it’s because of my link wich is not good?
Here is my code:
<a class="image-wrap" href="<?php echo $lightbox; ?>" rel="lightbox[gallery]" title="<?php the_title();?>"><?php the_post_thumbnail('alt-post-thumbnail'); ?><span class="zoom-icon"></span></a>

$lightbox is the link variable defined in the custom field named lightbox-url

    <?php
      $custom = get_post_custom($post->ID);
      $lightbox = $custom["lightbox-url"][0];
    ?>