The large gap between your main image and captions is partly due to setting imageVAlign="TOP".
This positions the image at the top of the gallery's image area rather than centering it vertically (which would balance the space equally above and below the main image).

The fact there is space at all is due to the gallery's image area having a different aspect ratio to your main image.
If you reduced the height of your gallery (try a fixed height such as '800px' or maybe '600px'), you should see the gap reduced, too.
Please see this FAQ for more details:
My Juicebox gallery shows too much space above or below the main image, how do I fix this?

Looking at the layout in your screenshot, setting captionPosition="OVERLAY" might also be a suitable compromise.

I hope one of these suggestions helps.

It would likely prove to be difficult to reposition the caption area by applying custom CSS to Juicebox's internal classes. As well as the difficulty in achieving the desired effect, Juicebox would have no knowledge of this modification and it might have unwanted, unforeseen knock-on effects (for example if the browser window is resized and the gallery needs to be redrawn). Also, custom modifications are not guaranteed to work with future versions of Juicebox.
I would recommend that you configure the gallery using the available configuration options. You might not be able to layout your gallery exactly as you like but you are less likely to run into problems.