[Plugin: Yet Another Related Posts Plugin] how to use the_post_thumbnail

You can do this using the YARPP templates feature. There’s a bundled template, in fact, which can do this. More info here: https://mitcho.com/blog/projects/yarpp-3-templates/

i saw those ones
the problem is that they are not getting the image at the_post_thumbnail, they are showing one wich is declared on the custom fields
and i need to show the one used on the_post_thumbnail
any idea?

The template yarpp-template-thumbnail.php is indeed using the_post_thumbnail().

I had to rework the included thumbnail template to get it to work right with my setup. When you upload your image are you selecting the “use as featured image” button instead of “insert into post”? I ended up doing it this way and then inserting the_post_thumbnail() into single.php where I wanted the related thumbnails to show up. Maybe that will help with which images get pulled?