Hi Nelly
OK, I think I've got it.
Toolset doesn't include a lightbox, but WordPress itself uses Thickbox. It's not the fanciest of lightbox implementations, and you may have a preference for another. I'll describe the solution using thickbox, and if necessary you can adapt it for another lightbox, the principles should be the same.
Any lightbox solution will need its JavaScript and CSS assets enqueuing. With thickbox that is pretty easy because WordPress has a function for that, otherwise you will need to enqueue the assets yourself (see https://developer.wordpress.org/themes/basics/including-css-javascript/).
With thickbox you can add a function like this to enqueue the files (e.g. as a snippet at Toolset > Settings > Custom Code):
add_action('template_redirect', 'enqueue_thickbox');
function enqueue_thickbox() {
if ( true ){
add_thickbox();
}
}
(I've added a meaningless condition to that code—if true—which you might want to replace with conditions for which pages the lightbox files are enqueued on, e.g. using is_page (https://developer.wordpress.org/reference/functions/is_page/).
Now to the View that sets up displaying lightboxes of the custom fields when the featured images are clicked on.
You are using a conditional shortcode to only show featured images of posts which have a value set for the image fields, and I think you have that working. (On my test site I'm using a simpler solution based upon the fact that if you order by a custom field and there is no value for that custom field, the post will be omitted from the results.)
Now to output of the View:
<wpv-loop>
<a href='[wpv-post-featured-image size="large" output="url"]' class="thickbox" rel="group-[wpv-post-id]">
[wpv-post-featured-image]
</a>
[wpv-for-each field='wpcf-slides']
<a class="thickbox" href="[types field='slides' output='raw'][/types]" rel="group-[wpv-post-id]">
</a>
[/wpv-for-each]
</wpv-loop>
The requirements for thickbox are that you output an image which itself is wrapped in a link to a full-sized version of the image and which has the class "thickbox". To group images into a gallery that can be navigated you also need to add the same rel attribute to all of the images that should be in the same group.
So, you can see how the featured image is output, and a rel attribute is dynamically generated (using the current post ID) for each post so that when we also add the custom field images they will be grouped together with their matching featured image.
The next part, using the wpv-for-each shortcode, is used to iterate over each image added as a custom field to a post (because the image field can have multiple values).
Note that we have to specify the normal Types prefix of "wpcf-" when specifying the custom field (which in my example has a slug of "slides").
Also, you don't want to show the custom field images initially, you only want to show them in the lightbox once the featured image has been clicked.
So I have added the links to the images, but the links themselves are empty, they do not contain the image tag itself, which I'm doing because the lightbox plugin requires the links for the lightbox to work.
I suggest you implement the same on your site with thickbox, and then if you want to try a different lightbox you then modify the above accordingly.
