The full screen button doesn't open the image in another lightbox, or rather a floatbox, but it is a script that activates the browsers native full screen, and expands the current view. fullscreen is used by several media sources like youtube for one. Everything you see with a fullscreen button uses it. There are a few different flavors out there, but for the most part they all do the same thing. Unfortunately; what you are wanting to achieve would require some serious code modification that would involve the Oxwall core. The only way to do that would be to hire a good programmer.