function auto_featured_image() { 
    global $post; 
 
    if (!has_post_thumbnail($post->ID)) { 
        $attached_image = get_children( "post_parent=$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); 
              } 
         } 
    } 
} 
// Use it temporary to generate all featured images 
add_action('the_post', 'auto_featured_image'); 
// Used for new posts 
add_action('save_post', 'auto_featured_image'); 
add_action('draft_to_publish', 'auto_featured_image'); 
add_action('new_to_publish', 'auto_featured_image'); 
add_action('pending_to_publish', 'auto_featured_image'); 
add_action('future_to_publish', 'auto_featured_image');