The first step would probably be to set enableDirectLinks="TRUE" (in JuiceboxBuilder-Pro's 'Customize -> General' section) so that each image in the gallery has its own unique URL in the form: http://www.example.com/gallery/#7 where the number after the # is the image number in the gallery.
A brief description of enableDirectLinks can be found in the General section of the Config Options page.
The next step would likely be to enable Google Analytics to allow anchor tracking by including location.hash in your Google Analytics code.
Try a web search with terms such as 'google analytics anchor tracking' for more information.
I cannot be sure that this will work but I hope it at least points you in the right direction.