User Level: 
Advanced

File migrations are a bit tricky. Files get their own class and registration, as if they themselves were a node like a Book Page. Basically, files have to be imported separately and then mapped to the content type they belong to. Here is the file code you will need for a migration:

class osu_migrateFileMigration extends DrupalFile6Migration {
  /**
   * Constructor
   */
  public function __construct(array $arguments) {
    if ((!isset($arguments['destination_dir']) ||
      	  empty($arguments['destination_dir']) ||
          $arguments['destination_dir'] == 'public://')
          && isset($arguments['og_group_id'])
          && !empty($arguments['og_group_id']))
    {
      $group_name = osu_migrate_get_group_name($arguments['og_group_id']);
      if ($group_name) {
        $arguments['destination_dir'] = "public://{$group_name}";
      }
    }
    parent::__construct($arguments);
  }

  /**
   * Custom error handler
   */
  public function errorHandler($error_level, $message, $filename,
                  $line, $context)
  {
    // If a file fails to save, let the user know (If they want errors).
    if ($error_level && stristr($message, 'mkdir(): permission denied')) {
      $output = "Could not mkdir() in destination path: Permission denied";
      $this->saveMessage($message, MigrationBase::MESSAGE_ERROR);
      throw new MigrateException('ERROR: ' . $output, 
                MigrationBase::MESSAGE_ERROR);
    }
    else if ($error_level && stristr($message, 'failed to open stream')) {
      $this->saveMessage($message, MigrationBase::MESSAGE_NOTICE);
      throw new MigrateException('NOTICE: ' . $message,
                MigrationBase::MESSAGE_NOTICE);
    }
  }
}

This code can either go in a separate file (such as file.inc) or be included in node.inc. Next, you need to point the mapping to the file migration in node.inc like so:

$this->addFieldMapping('field_picture', 'field_photo')
     ->sourceMigration('File');

Of course, you are still free to use prepareRow() to manipulate file data as necessary.