Hey!
Not sure if the lightbox plugin is compatible with lazyloading. I think it isn't.
function lightbox_image_element($img, $element, $obj){
$attid = $element->attid;
$full_src = wp_get_attachment_url( $attid );
return '<a class="image-link" href="'.$full_src.'">'.$img.'</a>';
}
add_filter( 'lg_frontend_img', 'lightbox_image_element', 10, 3 );
Then in your javascript:
$(document).ready(function() {
$('.image-link').magnificPopup({type:'image'});
});
Of course you need to include all files of Magnific Popup as described here:
http://dimsemenov.com/plugins/magnific-popup/documentation.html