Due to the nature of how Facebook works (taking the share image from an og:image tag in the web page), it would be very difficult for a gallery which is displayed from a single web page (such as a Juicebox gallery) to have the share image correspond to the image being shared.
I still have the same problems "my users can't post and share the correct photo".
As I mentioned above, although the share image (the thumbnail displayed in the Facebook pop-up share window) will always be the same one, the link being shared will still point towards the correct image in the gallery (the image currently being displayed in the gallery when the Facebook button is clicked).
For Juicebox, you really need to think of the share image as representing the gallery as a whole.
With this in mind, you could change the og:image tag in your gallery's web page to have the share image be any image you like. It does not need to be an image in your gallery.
If you really want to have the share image correspond to the image being shared, then you would need to find a gallery type which creates a separate web page for each image (so that each page/image can have its own og:image entry).
The downside to this type of gallery would be that it would likely be a standalone set of web pages rather than something that you could embed in an existing web page alongside other content (like you can with Juicebox).
Ideally, you would need to find a gallery that not only creates a separate web page for each image but also one that has built-in Facebook support so that og:image entries are generated automatically. Otherwise, it might be quite a time-consuming task to add an og:image entry manually for each image in your gallery.
Unfortunately, off the top of my head, I do not know of such a gallery type and a web search might be helpful.