It sounds like you are describing Small Screen Mode.

On mobile (small screen) devices, if the gallery is embedded in a web page alongside other content, then Juicebox displays the Splash Page (an image placeholder for the gallery) which, when tapped, expands the gallery to fill the browser viewport (to give the main images as much room to be displayed as possible).
In Small Screen Mode, after tapping the Splash Page (or if the gallery is presented on a web page of its own and the Splash Page is not used), the user will be presented with a grid of thumbnails from which a main image can be selected. (I think it might be this grid of thumbnails that you are referring to.)
When a thumbnail is tapped, the thumbnail page is replaced with the chosen image. (In Small Screen Mode, the thumbnails and main images are displayed on different pages to maximize the area available for the images on small screen devices.)
The user can return to the thumbnail page by tapping the 'Thumbnail' button on the gallery's Button Bar (and can return to the original embedding page by tapping the 'Close Gallery' button).
Please see the Screen Modes support section in the Gallery Tour for more information.

You can choose to not use the Splash Page by setting showSplashPage="NEVER" ('Customize -> Splash Page' section). Using this setting, the thumbnail page would initially be displayed instead of the Splash Page on small screen devices. If you want to skip the thumbnail page and go straight to the first main image in the gallery, then set showSmallThumbsOnLoad="FALSE" ('Customize -> Thumbnails').

Alternatively, you could force the gallery to be displayed in Large Screen Mode (which, by default, does not use the Splash Page and displays the thumbnails and main images together) on all devices and in all browsers by setting screenMode="LARGE" ('Customize -> General').

I hope this helps to clarify things and gives you a few options to try.