You can not. You must use the bigpopup window or not. You can turn lightbox on of off as needed as well.
Posting on lightbox support is probally best since they would be the ones to provide support for their code. As you can see if you view the source of your page we code lightbox into Photopost the same way they say. We include a css file a
js file and then simply the link tags have rel=lightbox and that means lightbox should work but not on your site for some reason.
Lightbox JS