I'm sorry to hear that you are having trouble with Juicebox-Pro.
Hopefully my notes below will help.

First of all, thank you for proving the link to your gallery's web page. Being able to see the problem live on your web server makes troubleshooting much easier than it would be otherwise.

The problem is caused by custom CSS from your 'dprl/sites/default/files/css/css_QCv8MsId5filqsoIiRNQ1kr6cc8cfMD65mL0h8pDWos.css' file.
Specifically, within this file, you have the following CSS:

#content img, #sidebar-first img, #sidebar-second img {
    max-width: 100%;
    height: auto;
}

These max-width and height CSS rules are being applied to all <img> tags (i.e. al images) within your #content container (which includes your Juicebox gallery).
The Juicebox gallery has no option but to inherit such global CSS rules. (It is the max-width CSS rule which is causing the problem and changing the width of the thumbnails within their frames.)

One possible solution would be to apply the custom CSS rules to only those images on your web page which require them through use of CSS selectors (ids and classes).

Another solution would be to try to protect the images within your Juicebox gallery from the custom CSS rule which is causing the problem.
Try adding the following CSS to your Juicebox gallery's '/dprl/sites/default/files/jbcore/classic/theme.css' file.

.juicebox-gallery img, .jb-idx-thb-list .jb-idx-thumb img, .jb-dt-main-frame img, .jb-idx-thumbnail-container .jb-splash-holder img {
    max-width: none !important;
}

I hope that this helps to resolve your problem.