The imageURL and thumbURL paths can be either absolute (starting with either http:// or https://) or relative (to the web page containing the gallery's embedding code).

If you use relative paths, then how the relative paths should be constructed will depend on the permalink structure that your WordPress site uses.

I would recommend using a leading slash in your paths to denote your root directory and then build up the relative paths from there.
For example, if you have an image located here:

http://www.example.com/wordpress/wp-content/uploads/2019/12/191011-rff-altria-0001-web-ready.jpg

... then you could use an imageURL such as:

imageURL="/wordpress/wp-content/uploads/2019/12/191011-rff-altria-0001-web-ready.jpg"

Otherwise, you could use an absolute path:

imageURL="http://www.example.com/wordpress/wp-content/uploads/2019/12/191011-rff-altria-0001-web-ready.jpg"

Both options above should work fine.
I hope this helps. Please let me know how you get on.

Thank you for posting the content of your XML file. I see the problem now.

The numerically-named XML file in the 'wp-content/uploads/juicebox' folder is not the XML file that Juicebox uses when the gallery is displayed. It simply stores the configuration options for the gallery (no image data) and other plugin settings.
Galleries created by WP-Juicebox do not use static XML files (like those generated by JuiceboxBuilder-Pro).
The gallery's actual XML data is generated dynamically at the time the gallery is displayed (from the 'config.php' file and functions within the 'wp-juicebox.php' file) using the options and settings stored in the numerically-named XML file.
It is not possible to add image data to the numerically-named XML file (and trying to adapt the plugin to display custom images would not be easy).
The plugin will always display all images attached to the post.

To display other images, you could either:
(1) Use Juicebox-Pro's Flickr options to display images from a Flickr account.
(2) Create a gallery on your computer with JuiceboxBuilder-Pro and embed the gallery manually in your post following the baseUrl method of embedding documented here (which I thought you might have initially been doing).

(1) Enter your gallery's baseUrl embedding code into a 'Formatting -> Custom HTML' Gutenberg block.

(2) Upload your complete gallery folder to your root directory and use a leading slash in your embedding code paths to denote your root directory. This should work with any permalink structure. For example:

<!--START JUICEBOX EMBED-->
<script src="/gallery_folder/jbcore/juicebox.js"></script>
<script>
    new juicebox({
        baseUrl: "/gallery_folder/",
        containerId: "juicebox-container",
        galleryWidth: "100%",
        galleryHeight: "600",
        backgroundColor: "#222222"
    });
</script>
<div id="juicebox-container"></div>
<!--END JUICEBOX EMBED-->

You can upload your gallery folder to any directory on your web server as long as the two paths in the embedding code (the path to the 'juicebox.js' and the baseUrl itself, pointing towards the gallery folder) are correct. (Your root directory might just be a convenient place for you to upload your gallery folder.)

I hope this helps.

If you do not want to use the plugin's functionality (of displaying all images attached to the post) and do not want to use Flickr as a source of images, then the best thing to do would be to forget about WP-Juicebox and create and embed a Juicebox gallery independent of the plugin.

Try the following:
(1) Create a gallery with JuiceboxBuilder-Pro on your computer and save it to a new and empty folder named 'gallery_folder'.
(2) Upload the complete gallery folder to the root directory of your web space.
(3) Enter the following embedding code (without modification) into a 'Formatting -> Custom HTML' Gutenberg block.

<!--START JUICEBOX EMBED-->
<script src="/gallery_folder/jbcore/juicebox.js"></script>
<script>
    new juicebox({
        baseUrl: "/gallery_folder/",
        containerId: "juicebox-container",
        galleryWidth: "100%",
        galleryHeight: "600",
        backgroundColor: "#222222"
    });
</script>
<div id="juicebox-container"></div>
<!--END JUICEBOX EMBED-->

As you have WP-Juicebox installed, you can use the 'jbcore' folder from the plugin if you like (to avoid the need to upload the one inside the gallery folder). Just change the path in the following line to point towards the 'wp-juicebox/jbcore/juicebox.js' file inside the plugin's folder. (This is not necessary but will save a little space on your web server.)

<script src="/gallery_folder/jbcore/juicebox.js"></script>

If you really want to, you can store your gallery images elsewhere (for example, your WordPress Media Library) but you'd need to adjust all the imageURL and thumbURL paths in your gallery's 'config.xml' file.
It would be much easier (and quicker) to just follow the three steps above. (No editing or modifications would be required.)

Please fill in the Download Link Request Form and we'll send you a new download link for your Juicebox-Pro purchase. (I cannot find your transaction under the email address you use for the forum.)

If you continue to experience difficulties, then please post the URL to your gallery's web page so that I can see the problem for myself.
Once I'm able to see your gallery live on your web server, I should hopefully be able to determine the exact cause of the problem and propose a solution.
Thank you.