Little Drupal 7 snippet for the image_link_formatter module (https://www.drupal.org/project/image_link_formatter) to add a fallback to lightbox in size large if no link is given. Should work the same with other fallbacks (colorbox, etc.). With some time you could create a clean field formatter from this containing the hard coded values as field display settings.
Please leave a short comment if this snippet was helpful for you.
?>
