Hocam function.php içerisine

add_filter('manage_posts_columns', 'posts_columns', 5);
add_action('manage_posts_custom_column', 'posts_custom_columns', 5, 2);
function posts_columns($defaults){
$defaults['riv_post_thumbs'] = __('Thumbs');
return $defaults;
}
function posts_custom_columns($column_name, $id){
if($column_name === 'riv_post_thumbs'){
echo the_post_thumbnail( 'featured-thumbnail' );
}
}

yapıştırın büyük ihtimalle olması lazım.