It looks like your problem may be due to a CSS conflict between some custom CSS code on your web page and the CSS code of the Juicebox gallery itself. Your web page may use some global CSS rules (which apply to all images on your web page including those in your Juicebox gallery) that the gallery has no option but to inherit. Unfortunately, it is not possible to isolate a Juicebox gallery (or any other element) on a web page to prevent it from inheriting global CSS rules.
Please check your CSS to see if you have any rules which are applied to all img tags within your web page's content section (which is where your Juicebox gallery is). It looks like you may have CSS rules which affect image attributes such as borders, margins/padding and possibly maximum-width.
Your web page loads 18 external CSS files so it might take a short while to search through them all but your browser's developer tools, usually accessible by pressing F12, should help. If you find any such rules, then please apply them to only those elements on your web page which require them through use of CSS selectors (ids and classes).