Ayuda - No se ven las miniaturas de mi theme

//Inicio utilizar primera imagen destacada
function auto_post_thumbnail() {		  global $post;	
	 $already_has_thumb = has_post_thumbnail($post->ID);
	 if (!$already_has_thumb)  {
	 	$attached_image = get_children( "post_parent=$post->ID&post_type=attachment&post_mime_type=image&numberposts=1" );
	 if ($attached_image) {								
	 foreach ($attached_image as $attachment_id => $attachment) {
	 	set_post_thumbnail($post->ID, $attachment_id);		
	 }				
	 } else {		
	 			set_post_thumbnail($post->ID, 'ID_imagen_por_defecto');
	 		}
	 							}
							} 
	 //Vamos añadir la función al loop
	 add_action('the_post', 'auto_post_thumbnail')
	 ;add_action('save_post', 'auto_post_thumbnail');
	 add_action('draft_to_publish', 'auto_post_thumbnail');
	 add_action('new_to_publish', 'auto_post_thumbnail');
	 add_action('pending_to_publish', 'auto_post_thumbnail');
	 add_action('future_to_publish', 'auto_post_thumbnail');
	 // Fin