Link Featured Image to Linked Media Image

image.php

<?php

// Create the variable for the large image src link
  $large_image_url = wp_get_attachment_image_src( get_post_thumbnail_id(), 'large' );
   
// Out the Post Title Using the post permalink for the post link - then the image which links to its large size
?>

  <h3><a href="<?php the_permalink(); ?>" rel="bookmark" title="<?php the_title_attribute(); ?>"><?php the_title(); ?></a></h3>

   <a class="lightboxclassname" href="<?php echo $large_image_url[0]; ?>" title="<?php the_title_attribute(); ?>" >

   <?php the_post_thumbnail('medium'); ?></a>