Have you set your medium threshold to 720 and actually rebuild mediums in scan database settings in admin? That is something entirely different than enabling lightbox.
The example your showing me has no medium image hense there is no need to use the big popupwindow or lightbox.
I looked around and that seems to be your issue your not creating medium images at so there is no large image.
This is an example of a medium image and you click it it to open lightbox or big popup window.
Pics - - ReefTalk Gallery
Now as previously discussed lightbox is only going to show an image up to say 1200 or 1500 without crashing but looks like to me if you set a medium size and build the mediums you should get something like the example I am showing.