Juicebox-Pro does not feature such functionality.

You could perhaps set it up manually but it could turn out to be a complex task and programming knowledge (HTML, JavaScript and PHP) would likely be required.

You would need to add a guest book to the web page containing your gallery and this would require a server-side scripting language (such as PHP) to store the comments in a database or file on your server. Try a web search with terms such as 'PHP guest book' to get you started.

If you wanted the comments to be specific to a particular image in a gallery (rather than just relating to the gallery as a whole), then you could determine the current image being displayed in the gallery using the Juicebox-Pro API (specifically the getImageIndex() method) and then perhaps somehow integrate this into the guest book you choose to use (if it is flexible enough to allow such modifications).

It would not be a quick or easy task (as Juicebox-Pro was not designed with this in mind) but I hope this points you in the right direction.

Maybe you could build a web site with WordPress. You could create your galleries with the WP-Juicebox plugin and just use WordPress's own built-in commenting system (although the comments would be per page or post rather than per image in a gallery).