Here’s a code I use in the single.php to show the thumbnails of other posts in the same category.
Might help you started 🙂
$category = get_the_category(); //get first current category ID
$this_post = $post->ID; // get ID of current post
$posts = get_posts('numberposts=500&orderby=rand&category=' . $category[0]->cat_ID . '&exclude=' . $this_post); ?>
<?php foreach($posts as $post) { ?>
<a href="<?php the_permalink() ?>" title="<?php the_title(); ?>">
<?php the_post_thumbnail( array(150, 150) ); ?>
</a>
<?php } wp_reset_postdata(); ?>
</div>