So far, I see a part of the first photo in the viewer that is cropped on all sides.
Maybe you're just seeing the Splash Page (see here for details and scroll down).
The Splash Page is an image placeholder for the gallery which, when clicked, will expand the gallery to fill the browser viewport (to give the gallery as much room as possible).
By default, the Splash Page will be displayed on mobile devices when a gallery is embedded in a web page alongside other content (where the gallery might be too small to use). By default, Juicebox uses the first image in the gallery as the Splash Page image (and crops if necessary). You can change this if you like via the splashImageUrl option (in JuiceboxBuilder-Pro's 'Customize -> Splash Page' section).
You could also choose to not use the Splash Page at all by setting showSplashPage="NEVER".
Please note that these are Pro options (not available in Juicebox-Lite).
Also, if you are using a fixed value for your galleryWidth, try using a percentage instead (e.g. 100%).
However, please be sure to set widths for all parent containers using CSS so that your browser knows what the gallery's width should be 100% of.