Found a solution:
In includes/classes/display_images.php:
replace:
<script language="javascript"><!--
document.write(\'<a href="' . (!$this->onpagemenu ? 'java script:popupWindow(\\\'' : '') . tep_href_link( $this->linkurl, ($item==0&&$this->groupwithparent=='true'?'pID='.$products_id:'imagesID='.$addimages_images[$item]['id'])) . (!empty($_GET['products_id']) ? '&products_id='.$_GET['products_id'] : '') . (!empty($_GET['cPath']) ? '&cPath='.$_GET['cPath'] : '') . (!$this->onpagemenu ? '\\\')' : '') . '"' . $t_menu_mouseover . '>' . tep_image(DIR_WS_IMAGES . $addimages_images[$item]['image'], addslashes($addimages_images[$item]['desc']), (ADDIMAGES_RESTRICT_PARENT=='false'&&$item==0&&$this->groupwithparent=='true'?'':$addimages_image_width), (ADDIMAGES_RESTRICT_PARENT=='false'&&$item==0&&$this->groupwithparent=='true'?'':$addimages_image_height), 'hspace="5" vspace="5"') . '<br>' . (!empty($addimages_images[$item]['desc'])?addslashes($addimages_images[$item]['desc']):TEXT_CLICK_TO_ENLARGE) . '</a>\');
//--></script>
with
<script language="javascript"><!--
document.write(\'<a href="images/' . $addimages_images[$item]['image'] . '"target="_blank" rel="lightbox[group]" title="' . addslashes($addimages_images[$item]['desc']) .'">' . tep_image(DIR_WS_IMAGES . $addimages_images[$item]['image'], addslashes($addimages_images[$item]['desc']), (ADDIMAGES_RESTRICT_PARENT=='false'&&$item==0&&$this->groupwithparent=='true'?'':$addimages_image_width), (ADDIMAGES_RESTRICT_PARENT=='false'&&$item==0&&$this->groupwithparent=='true'?'':$addimages_image_height), 'hspace="5" vspace="5"'). '</a>\');
//--></script>
In product_info.php use
<script language="javascript"><!--
document.write('<?php echo '<a href="images/' . $product_info['products_image'] . '"target="_blank" rel="lightbox[group]" title="' . $product_info['products_name'] . '">' . tep_image(DIR_WS_IMAGES . $product_info['products_image'], addslashes($product_info['products_name']), SMALL_IMAGE_WIDTH, SMALL_IMAGE_HEIGHT, 'hspace="5" vspace="5"') . '<br>' . TEXT_CLICK_TO_ENLARGE . '</a>'; ?>');
//--></script>
The
rel="lightbox[group]"
differs. All images will now show in the lightbox-popup (image 1 of........etc)