bbPress.org

Add the following code to your functions.php

function forum_icons() {
	if ( 'forum' == get_post_type() ) {
		global $post;
	    if ( has_post_thumbnail($post->ID) )
	    	echo get_the_post_thumbnail($post->ID,'thumbnail',array('class' => 'alignleft'));
	 }
}
add_action('bbp_theme_before_forum_title','forum_icons');

Hope it helps 😉