En configuration locale :

<?php

/* $conf['original_url_protection'] = 'images' */

function addfbtpl($content, &$smarty)
{

   $search = '/<\/head>/';
 
  $replacement = '<meta property="og:title" content="{$GALLERY_TITLE} - {$ALT_IMG}">
<meta property="og:description" content="{$COMMENT_IMG}">
<meta property="og:image" content="'.get_absolute_root_url().'{$current.THUMB_SRC}">
</head>
';
 
  return preg_replace($search, $replacement, $content);
 
}







?>

et en onglet plugin personnel  :
<?php
/*
Plugin Name: Plugin Personnel
Version: 1.0
Description: Plugin Personnel
Plugin URI: http://piwigo.org
Author:
Author URI:
*/




?>

Ce sont des éléments que j'ai glané sur le forum