Your gallery (the 'juicebox-container' <div>) is horizontally centered within its parent container and, within the gallery, the thumbnails are centered within the thumbnail area and the main image is centered within the image area. Most of your main images are portrait style and, in a landscape style gallery, there is space to the left and right of the main image. Try resizing your gallery browser (by reducing its width) and you will see that the thumbnails and main images become closer together.
the thumbnails are flush with the left side of the container
This is because you have set thumbsPosition="LEFT" and thumbNavPosition="BOTTOM".
the image with the largest horizontal measurement doesn't come close to the right side
Currently, there is no configuration option available to horizontally align the main images to the right edge of the image area. However, in a browser window of a certain aspect ratio (closer to portrait than landscape), the right side of your main images will be flush against the right edge of the gallery. Try resizing your browser window and you should see this.
And I would actually like to reduce the distance between the thumbnails and the main image, if that's at all possible.
My suggestions above would move the thumbnails away from the left edge of the gallery.
If you wanted the thumbnails as close to the main images as possible (for images of all aspect ratios), you could set the following configuration options:
thumbsPosition="LEFT"
maxThumbRows="3"
maxThumbColumns="3"
thumbNavPosition="BOTTOM"
thumbPadding="0"
imagePadding="0"
imageScaleMode="FILL"
Also, changing the thumbNavPosition also made the entire gallery jump from one position to the other, but still slewing left.
Changing thumbNavPosition from "BOTTOM" to "CENTER" will reserve space to the left and right of the thumbnails for the thumbnail navigation arrows.
As I noted above, there are currently no configuration options to horizontally and vertically align the thumbnails and main images within their respective areas but such options should be introduced in the next version of Juicebox-Pro.