Only use a specific type of page that you don't intend to be accessed directly, just in the lightbox. Like forms or help dialogs. In your theme or child theme you need to create a page template that has nothing but the content. Do not include header or footer, do not include any fancy layout, just what is necessary - a basic HTML skeleton. It should look pretty raw. On your pages, select this as "Template". In your JIG instance, add an image that has the JIG Link set to this page and JIG Target set to lightbox. Recent posts is not needed because you have custom links on the images. This'll make your post content load in the lightbox that supports iframes.