<?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; ?>