The private gallery is for you to store images that are not accessible by other members. At the moment, there is no way to allow certain memebers to view your private gallery. However this is something we're working on.
When you add images to a post, you have a filter setting to allow you to view images uploaded by "Self" or "Everyone". If you click "Self", you can pick images from your private gallery.
The reason behind this whole thing is so members dont' have to clutter the gallery with pictures that they only want to use in threads or blog posts. It also means you have a single place to review all the images you've used in your posted content.
~Paul