My thinking Bill, is that people don't always move around in straight lines. :)  So people go to thumbnails that are not in order.  This means that they have to wait each time for the image to load.  I have my thumbnails set to display the image on Hover, this combined with pre-load all makes for a very fluid experience ... once the initial load is done.  Also, the first page of images are there very fast and the others are fully downloaded by the time a user would navigate to the next page. 

My own feeling is that I'm happy to wait for a site to load, but once that is finished I don't want a stuttering experience.

Thanks Steven.  I hadn't clicked on the 'Change Sizes' button, so didn't see where this feature was located.

As far as I am aware, the largeImageURLs will be used for the main display in the gallery if the display is retina and Large Screen Mode is being used. If screenMode="AUTO", Large Screen Mode will be used only if the device is not a small-screen mobile device (for example an iPhone) and the screen width is greater than 1000px.

What device are you using? Perhaps it is classified (incorrectly) within Juicebox as a small-screen device. If you let me know what device you use, I will investigate further for you.
Have you tried setting screenMode="LARGE" (in JuiceboxBuilder-Pro's 'Customize -> General' section) to see if this makes a difference?

Juicebox determines whether the screen being used to display the gallery is a retina display on the information provided by devicePixelRatio on the JavaScript window object. However, Juicebox chooses the Screen Mode (when screenMode="AUTO") on a number of factors, one of which is the device in use. Both a retina display and Large Screen Mode are required for largeImageUrls to be used for the main images in a gallery.
What result is returned when you run this snippet of JavaScript code in an HTML page with your external monitor attached? (A retina display should return a value of 2.)

<script>
    alert(window.devicePixelRatio);
</script>

@martinkulhavy78

Thank you for reporting this issue.
I have notified the developers who will investigate further.

Thanks for your help.
I've been able to replicate the problem in a test gallery of my own and a bug report has now been logged.
Hopefully the problem will be fixed for the next version.

Unfortunately, I really do not know when this issue will be fixed or when the next version of Juicebox will be released.
There is no set release schedule and any estimate I give you could turn out to be wholly inaccurate.
It is unlikely to be a matter of days but if I said it might be a few weeks, it could turn out to be a few months (or vice versa).
I'm sorry I can't give you a better idea of how long you might have to wait.

If it's any consolation, Multi-Size Image support is not completely dysfunctional and still works OK on Apple-branded 'Retina Display' devices (both mobile and desktop), where the term 'retina' refers specifically to Apple's 'Retina Device' trademark.
However, Juicebox-Pro is not currently recognizing other, non-Apple, high-density pixel ratio devices which is the nature of the problem and the reason for the bug report.