After installing the 'Pin It' button in Firefox, viewing a demo Juicebox gallery such as http://www.juicebox.net/demos/pro/full/
... and clicking  the 'Pin It' button, the currently displayed main image, previous main image and next main image are all found by Pinterest (along with all the thumbnails), which should at least allow users to Pin the main image which is visible when clicking the 'Pin It' button.
Changing the imagePreloading configuration option does not seem to make a difference.
It looks like it would not be possible to enable the 'Pin It' button to find all main images in a Juicebox gallery (without modification to the 'Pin It' code).