Thank you @netweb .
I just added the permalink, title and change the size from thumbnail to 80×80:
add_action( 'widgets_init', 'rkk_widgets_init' );
add_post_type_support('forum', array('thumbnail'));
function ks_forum_icons() {
if ( 'forum' == get_post_type() ) {
global $post;
if ( has_post_thumbnail($post->ID) )
echo '<a href="' . get_permalink( $_post->ID ) . '" title="' . get_the_title( $_post->ID ) . '">';
echo get_the_post_thumbnail($post->ID,array(80,80),array('class' => 'alignleft forum-icon'));
echo '</a>';
}
}
add_action('bbp_theme_before_forum_title','ks_forum_icons');
🙂