Display the post only if thumbnail is set up

I think I have successfully been able to display the last 3 thumbnails of a set of subpages in chronological order.

Now I’d like to add a conditional statement that will only display the post if the page has a thumbnail set up. I’m a novice with PHP. I’d appreciate the help.

This is the current code.

<?php query_posts( array(
'post_parent' => 12,
'post_type' => 'page',
'posts_per_page' => 3,) );
?>

<?php if (have_posts()) : while (have_posts()) : the_post(); ?>

<div class="work-item"><?php the_post_thumbnail(); the_title(); ?>
  <a href="<?php the_permalink() ?>">View Details</a>
</div>

<?php endwhile; endif; ?>