hi @Chris
use the full parameters for your get_field, means:
$related_1 = get_field( '1_related_product', false, false );
this make ACF get the values from this post.
and as @Elliot said, use:
echo get_the_post_thumbnail( $related_1, 'thumbnail' );
to echo the image size you want.
let me know anything happened.
A