It’s not a plugin but a manual implementation of the Fancybox 1.3.4.
if ( $.isFunction($.fn.fancybox) ) {
$('a:not(.post-edit-link)').fancybox({
'type': 'ajax',
'ajax': {
dataFilter: function(data) {
return $(data).find('#content').html();
}
},
'onComplete' : function( links, index ) {
}
});
}
The above finds all single post content within the element id #content.
the_post_thumbnail is called within the single page template along with the_content and separately stored gallery shortcode information (hence why the gallery loads without any injection from this plugin).
The ideal solution would be to prevent the plugin injecting data-lazy-original in the first place or to somehow activate the javascript function to carry out the image replacement
h.src = h.getAttribute("data-lazy-original");
h.removeAttribute("data-lazy-original")