You can set showThumbsOnLoad="FALSE" (for when the gallery is displayed in Large Screen Mode) and showSmallThumbsOnLoad="FALSE" (for Small Screen Mode) to hide the thumbnails when the gallery is initially loaded.
Please note that both of these configuration options are Juicebox-Pro configuration options only which, unfortunately, are not supported by Juicebox-Lite (the free version).
If you are using Juicebox-Pro, then these configuration options can be found in JuiceboxBuilder-Pro's 'Customize -> Thumbnails' section and, if you are using the Drupal module, then Pro configuration options can be entered entered into the 'Juicebox Library - Pro / Manual Config' section of your Drupal Dashboard ('Home -> Administration -> Structure -> Content types -> Article -> Manage Display -> Juicebox settings').