Instead of my original suggestion above, try adding the following to any of your CSS files:

.jcbx-glry-classic {
    direction: ltr;
}

This should hopefully work for both normal and fullscreen modes.

I have logged this issue as a bug with the developers and it should hopefully be fixed in a future version of Juicebox.
Thank you for reporting it.

I'm glad you've been able to resolve your problem.
Thank you for posting back to let me know.

As far as I can remember, I have not encountered such a problem before and, unfortunately, a screenshot would not help me troubleshoot it.
I would really need to see the gallery live on your web server (so that I can check the code on the web page and the gallery's configuration options).
If possible, please post the URL to your gallery's web page so that I can take a look and the problem for myself and hopefully help further.
Otherwise, please post your gallery's configuration options so that I can try to replicate the problem in a test gallery of my own.

Also, if you are not already using the latest version of Juicebox-Pro (v1.4.4.1), then please try upgrading your gallery to see if this helps. (Your problem may be caused by a bug in a previous version which has since been fixed.)
Full instructions for downloading the latest version and upgrading existing galleries can be found here.

I'm glad you've been able to resolve your problem.
Thank you for posting back to let me know.

For others experiencing similar issues, here's a link to a support page detailing the <a> tag's 'target' attribute with its possible values.

@junebugweddings

You could set an image title and/or caption for the last image in your gallery and have it include links (<a> tags) using HTML formatting as noted in this FAQ:
How do I add HTML formatting to image captions and titles?

Alternatively, you could use the Juicebox-Pro API to check whether the last image in the gallery is currently being displayed and run some custom JavaScript code (to do whatever you like) if it is.
For example:

<!--START JUICEBOX EMBED-->
<script src="jbcore/juicebox.js"></script>
<script>
var jb = new juicebox({
    containerId: 'juicebox-container'
});
jb.onImageChange = function(e) {
    if (e.id === jb.getImageCount()) {
        alert('Last image in gallery!');
    }
};
</script>
<div id="juicebox-container"></div>
<!--END JUICEBOX EMBED-->

Set showSmallBackButton="TRUE" (in JuiceboxBuilder-Pro's 'Customize -> Back Button' section). (The default value for this configuration option is FALSE.)

Your two suggestions seem to be based around dynamically creating a new page for each image with the required og:image tag. This might work to ensure that the corresponding thumbnail is displayed each time an image in the gallery is shared but then the page that is shared will no longer be the page containing the Juicebox gallery. It will be your custom page with the required og:image. I'm not sure if this is a drawback or something that is intended. (Maybe you could redirect back to the gallery page from the page containing the og:image.) Just thinking out loud...

You're welcome!
I'm glad you've got it working and are getting on well with Juicebox.
Thank you for posting back to let me know.

Your gallery's embedding code uses the following line:

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

Therefore, your gallery will display if you copy the contents of your gallery folder (not the actual folder itself) into the same directory as the web page containing the embedding code.

If you have uploaded your entire gallery folder to your web server, then you can use the baseUrl method of embedding documented here.

If you get stuck, please let me know where your gallery files are on your web server and I should be able to provide you with some embedding code that will work.
Thank you.

I'm glad you've got things working.
Thank you for posting back to let me know.

The first step would probably be to set enableDirectLinks="TRUE" (in JuiceboxBuilder-Pro's 'Customize -> General' section) so that each image in the gallery has its own unique URL in the form: http://www.example.com/gallery/#7 where the number after the # is the image number in the gallery.
A brief description of enableDirectLinks can be found in the General section of the Config Options page.

The next step would likely be to enable Google Analytics to allow anchor tracking by including location.hash in your Google Analytics code.
Try a web search with terms such as 'google analytics anchor tracking' for more information.

I cannot be sure that this will work but I hope it at least points you in the right direction.

Are you perhaps trying to view your gallery using http://joaqimb.be instead of http://www.joaqimb.be?
You have hardcoded the 'www' subdomain into the paths in your embedding code so your gallery will display only when using the 'www' subdomain.
Please see this FAQ for details and a solution.
My gallery works on 'www.example.com' but not on 'example.com' (or vice versa). Why?

In your case, paths such as the following should work fine:

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

baseUrl: "/JbMuzeuml/",

(The leading slash in the paths denotes your root directory.)

Hopefully this will solve your problem.