SOLVED!
I delete the default_images_additional.php in template/html/com_virtuemart/productdetail/ and left the original one installed with virtuemart. It works! here the code of the original one:
<?php
/**
*
* Show the product details page
*
* @package VirtueMart
* @subpackage
* @author Max Milbers, Valerie Isaksen
* @link
http://www.virtuemart.net * @copyright Copyright (c) 2004 - 2010 VirtueMart Team. All rights reserved.
* @license
http://www.gnu.org/copyleft/gpl.html GNU/GPL, see LICENSE.php
* VirtueMart is free software. This version may have been modified pursuant
* to the GNU General Public License, and as distributed it includes or
* is derivative of works licensed under the GNU General Public License or
* other free or open source software licenses.
* @version $Id: default_images.php 7784 2014-03-25 00:18:44Z Milbo $
*/
// Check to ensure this file is included in Joomla!
defined('_JEXEC') or die('Restricted access');
?>
<div class="additional-images">
<?php
$start_image = VmConfig::get('add_img_main', 1) ? 0 : 1;
for ($i = $start_image; $i < count($this->product->images); $i++) :
$image = $this->product->images[$i];
?>
<div class="floatleft">
<?php
if(VmConfig::get('add_img_main', 1)) {
echo $image->displayMediaThumb('class="product-image" style="cursor: pointer"',false,$image->file_description);
echo '<a href="'. $image->file_url .'" class="product-image image-'. $i .'" style="display:none;" title="'. $image->file_meta .'" rel="vm-additional-images"></a>';
} else {
echo $image->displayMediaThumb("",true,"rel='vm-additional-images'",true,$image->file_description);
}
?></div>
<?php endfor; ?>
<div class="clear"></div>
</div>