Instead of linking a featured image to a post permalink in a loop – here is how you can link to its original image of itself.
|// 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; ?>" title="<?php the_title_attribute(); ?>" >|
|<?php the_post_thumbnail('medium'); ?></a>|
So above, the title and featured image is output of a post – the title has a link to the post whilst the image is linked to itself via the $large_image_url which uses the first value in the array  which is the src value.
Further more a lightbox CSS class name can be triggered so a bunch of featured images can be viewed in an archive loop page without going to the single post page