Here is the link to the prettyPhoto lightbox plugin website –
but the markup is as simple as having a link with an extra attribute rel=”prettyPhoto” and it should link to an image URL. The content of the link can be anything – text(like Preview text below), image (img element) etc.
I see that the prettyPhoto lightbox script is not being loaded in single portfolio pages for the Extinct theme (since we try to optimize the page load by not loading unwanted scripts). You can load the same by modify the line number 537 of framework/framework.php file to remove the if condition or have the below condition
if (is_archive() || is_singular(array('post', 'page', 'portfolio'))) wp_enqueue_script('jquery-prettyphoto', MO_SCRIPTS_LIB_URL . '/jquery.prettyPhoto.js', array('jquery'), '3.1.4', true);
to help load the prettyPhoto script which helps load the image in the lightbox.