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The Function:

function featuredImageLightbox($thumbSize) {
  ?>
  <a rel="lightbox" href="<?php $image_id = get_post_thumbnail_id();
  $image_url = wp_get_attachment_image_src($image_id, 'full', true);
  echo $image_url[0];
  ?>">
    <?php the_post_thumbnail($thumbSize); ?> </a>
<?php
}

Usage:

<?php featuredImageLightbox('medium'); ?>