Modifying destination directory for imagefield file migrating D6->D7
To avoid future situations like this, here is what I came up with:

  public function prepare($entity, stdClass $row) {
    // Change the destination directory for the field_mainphoto and
    // field_thumbmailphoto files to be /images.
    foreach (array('main_photos', 'thumbnail_photos', 'video_thumbnails') as $dir) {
      if (substr($entity-&gt;destination_file, 1, strlen($dir)) == $dir) {
        $entity-&gt;destination_file = str_replace($dir,'images', $entity-&gt;destination_file);
      }
    }   
  }