I had really hoped that I could stretch the image and the thumbnail gallery to the edges of the page, but I guess the sides of the thumbnail gallery are just inaccessable.
You could use imageScaleMode="FILL" to completely fill the image area with the main image (though cropping may occur on large images). However, the thumbnails are of fixed dimensions and are centered within the gallery and the gallery is embedded in a container with a fixed width on your WordPress page. If the container was slightly narrower that it currently is, there would be less room to the left and right of the thumbnails but the container size is set by your WordPress theme. You could check your WordPress theme settings to see if you could decrease the post container width.
As one last tweak, Steven, do you see any way that I can minimize the margin at the bottom bottom of my gallery?
Since you have set thumbNavPosition="BOTTOM", Juicebox is reserving space for the thumbnail navigation buttons below the thumbnails.
If the gallery was embedded in a container which did not have a fixed width and the user decreased the width of the browser window, the number of thumbnails per page would decrease and the thumbnail navigation buttons would appear in this space to allow the user to navigate through the thumbnail pages.
If you find a way to change your post container's width in your WordPress theme settings, you might like to try increasing the post container's width slightly and reverting to setting thumbNavPosition="CENTER".