There is a bug in the buildPhotoURL method. I've already fixed it for the next update. If you want to use this method, here's the simple fix:
Change Line 182 in phpFlickr.php from:
$url = "http://photos" . $photo['server_id'] . ".flickr.com/" . $photo['photo_id'] . "_" . $photo['secret'];
To:
$url = "http://photos" . $photo['server'] . ".flickr.com/" . $photo['id'] […]