Enfold Photography – original quality picture in full screen slider/

Please have a look at your source code to see if you really get the full resolution of the Lightbox pictures.

To set the source for the thumbnails there is, as far as I know, no filter that would make our work easier.
But you can put a copy of the av-helper-masonry.php in the Child-Theme folder ( best in a subfolder: shortcodes ), and then edit this copy.

search for the function prepare_loop_from_entries
( on Enfold 4.6.2 it is line : 601) – you will find on 612 :

$img_size	 = 'masonry';
//change to :
 $img_size	 = 'full';

to load the edited ALB elements put this to child-theme functions.php

add_filter('avia_load_shortcodes', 'avia_include_shortcode_template', 15, 1);
function avia_include_shortcode_template($paths){
  $template_url = get_stylesheet_directory();
      array_unshift($paths, $template_url.'/shortcodes/');
  return $paths;
}

refresh all cachings – merging – performance tools.
Look to your page.