When using the Media Library as the source of images, WP-Juicebox displays all images attached to the post or page containing the gallery.
Unfortunately, there is no way to exclude certain images (other than the featured image). WP-Juicebox does not do any image handling itself. It relies solely on WordPress's own Media Library functionality. If WordPress natively supported tagging Media Library images, it would make the ability to exclude images from a WP-Juicebox gallery much easier to implement. There are third-party plugins available which allow the tagging of Media Library images but not everyone will use the same one (and they all implement tagging slightly differently) so there's no easy way to do this.

For more control over which images are displayed in your galleries, you might like to use a different image source, such as a Flickr account.
You could upload all your images to Flickr, tag them appropriately (in the Flickr interface) and tell WP-Juicebox which images to display via the 'Flickr Tags' configuration option.
That would probably be the easiest way to achieve what you are looking to do.

Would you happen to know how I can hide the gallery title. I don't want to not have one, but I don't want it to display... I was trying: galleryTitle="" but it didn't seem to work, and doesn't stay displayed in the Pro Options area.