well i figured it out - but i think that there could be a more beautiful solution - i post mine solution here, but looking forward to get any improvements:

i updated /includes/modules/additional_images.php

somewhere at line 92 - search for:
// Link Preparation:

and change the link with the javascrip popupWindow call to something like this:

$script_link = '<div style="width:150px;background-color:#eee;padding-top:5px;padding-bottom:5px;border-style:solid;border-width:1px;border-color:#ddd;"' . '<script language="javascript" type="text/javascript"><!--' . "\n" . 'document.write(\'' . ($flag_display_large ? '<a href=" ' . $products_image_large . '" class="fancy" rel="imageset"> ' . $thumb_slashes . '<br />' . TEXT_CLICK_TO_ENLARGE . '</a>' : $thumb_slashes) . '\');' . "\n" . '//--></script>';