bbPress.org

none of this testd, but the actual adding of an image to the post would be something like

add_action('bbp_template_before_single_topic', 'pym_print_featured_image_single_forum', 11);
		
	function pym_print_featured_image_single_forum() {
		if (has_post_thumbnail()) {
			echo '<div class="pym-bbp-topic-thumbnail">';
			echo get_the_post_thumbnail(null, 'thumbnail' );
			echo '</div>';
		}
	}

adding featured topic to the backend would be something like

add_post_type_support('topic', array('thumbnail'));

Code could be written to allow the user to add an image, but that’s a chunk of work !!