Just to clarify (for others reading this thread), the image currently being viewed in the gallery is the image being shared when the Facebook share icon is clicked.
The shared link has a # at the end of the URL to signify which image in the gallery is being shared and, when the shared link is clicked, the gallery will display the shared image (rather than just the first image in the gallery).
However, the thumbnail image displayed in the Facebook pop-up share window will be taken from the web page's og:image tag and, therefore, is not a thumbnail of the image being shared (it is just a single representative image for the gallery as a whole).

The only user I know of who seems to have managed to overcome this restriction is sly from this thread.
I notice that his solution seems to be included within a rather complex custom JavaScript file (check out the 'jcaddon.js' file on his gallery pages) which overrides some of Juicebox-Pro's own sharing functionality and, therefore, is not officially supported.
(As such, I have not tested his solution myself and I cannot be sure that it will work across all platforms and in all browsers.)
However, I hope this points you in the right direction.