Wait, see if this will work:
Set the post ID to $post_id inside your function. Then, use this:
<?php echo get_the_post_thumbnail($post_id, 'single-post-thumbnail'); ?>
Wait, see if this will work:
Set the post ID to $post_id inside your function. Then, use this:
<?php echo get_the_post_thumbnail($post_id, 'single-post-thumbnail'); ?>