This should do it:

global $post;
$ancestors = get_post_ancestors($post->ID);

$photos = get_children(array(
  \'post_mime_type\' => \'image\', 
  \'post_parent\'    => array_shift($ancestors),
));

$index = 0;
foreach($photos as $photo){
  $index++;
  if($photo->ID === $post->ID) break;
}

printf(\'%d out of %d\', $index, count($photos));