Without seeing the html output for the excerpt thumbnails it is difficult to say why your code is not working.
However I would also look into str_replace (http://php.net/manual/en/function.str-replace.php) as a possible solution to your problem.
Without seeing the html output for the excerpt thumbnails it is difficult to say why your code is not working.
However I would also look into str_replace (http://php.net/manual/en/function.str-replace.php) as a possible solution to your problem.