Problems with thumbnail sizes

I’m using the code below to display a list of posts on my homepage with thumbnail, title, and summary text.

The problem I’m having is that rather than using the thumbnail image which is a max size of 377px it’s using the full 1280px image and scaling in the browser.

<?php
	global $post;
	$tmp_post = $post;
	$args = array( 'numberposts' => 3, //'offset'=> 1,
	'category' => 67 );
	$myposts = get_posts( $args );
	foreach( $myposts as $post ) : setup_postdata($post); ?>

<li>
<?php echo get_the_post_thumbnail(); ?> 				<div class="post-details-wrapper">
<h3><a href="<?php the_permalink(); ?>"><?php the_title(); ?></a></h3>
<?php the_excerpt(); ?>
</div>
	</li>
	<?php endforeach; ?>
	<?php $post = $tmp_post; ?>

Now when I add ‘thumbnail’ ‘medium’ or any other attribute to get_the_post_thumbnail(); I don’t get any image at all.

Presumably I’m doing something wrong.