I modified the thumb.php in /wp-content/themes/montezuma/includes and I think it works. At least my site still works and no faulty featured images anymore.
I removed line 58 to 64.
} elseif ( bfa_has_attachment() ) {
$thumb = bfa_get_first_attachment_id();
$hasthumb = TRUE;
} elseif( bfa_has_img_src() ) {
$thumb = bfa_get_first_img_src();
$hassrc = TRUE;
and saved the thumb.php.