I have verified that this code works. Created a github repository just for this topic. Code and instructions are included.
https://github.com/ericwallen/add-featured-images-bbpress-forums
<!– Add this code to functions.php –>
add_theme_support( ‘post-thumbnails’, array( ‘forum’ ) );
add_post_type_support(‘forum’, ‘thumbnail’);
<!– Place this code inside page.php or anywhere else you can echo php in your theme –>
<?php echo get_the_post_thumbnail($post_id, ‘full’); ?>
<!– Repace $post_id with a number ’54’ which will be the id of the post you are tryin to fetch –>
<?php echo get_the_post_thumbnail(’54’, ‘full’); ?>