ok I ended up by accessing the db directly :
$db = JFactory::getDBO();
$query = $db->getQuery(true);
$query->
select('avatar')->
from('#__cck_store_item_users')->
where('id='.(int)$user->id);
$db->setQuery($query) ;
$avatar = $db->loadResult();
but this gives me the full path to the uploaded file : images/users/xxx/image_name.jpg
How can I get the thumbnail path? ( eg. images/users/xxx/_thumbx/image_name.jpg )