Post Thumbnail Linking to Large Image Size
<?php if ( has_post_thumbnail() ) : ?>
<a class="popup" href="<?php
$large_image_url = wp_get_attachment_image_src( get_post_thumbnail_id(), 'large' );
echo $large_image_url[0];
?>" title="<?php the_title_attribute(); ?>">
<?php the_post_thumbnail(array(250, 250)); ?>
</a>
<?php endif; ?>