how can I call a posts featured image in any template?

I think you are looking for get_the_post_thumbnail() which takes accepts the post ID as a parameter. get_the_post_thumbnail() generates the entire image code. There is no need for you to try to create your own <img> tag.

It is used as follows:

echo get_the_post_thumbnail( $post_id, $size, $attr );

So for your example, remove the following:

<img src="https://wordpress.stackexchange.com/questions/129429/<?php echo get_the_post_thumbnail("25, medium' ); ?>">

and replace with:

echo get_the_post_thumbnail( 25, 'medium' );