Thank you mesnitu.
Yes I got it.
Not being a coder I thought it was harder, thus I asked, thanks to your post I read the lines carefully, and to my surprise I understood it! ;)
For someone else needing it, I changed
CUSTOM_TEMPLATE/templates/tpl_modules_main_product_image.php
like this
Code:
<?php require(DIR_WS_MODULES . zen_get_module_directory(FILENAME_MAIN_PRODUCT_IMAGE)); ?>
<div id="productMainImage" class="centeredContent back">
<!-- Button trigger modal -->
<div type="button" class="btn btn-lg" data-toggle="modal" data-target="#myModal">
<?php
echo zen_image($products_image_medium, $products_name, MEDIUM_IMAGE_WIDTH, MEDIUM_IMAGE_HEIGHT) . '<span class="imgLink">' . TEXT_CLICK_TO_ENLARGE . '</span></a>';
?>
</div>
<!-- Modal -->
<div class="modal fade" id="myModal" tabindex="-1" role="dialog" aria-labelledby="myModalLabel">
<div class="modal-dialog" role="document">
<div class="modal-content">
<div class="modal-header">
<button type="button" class="close" data-dismiss="modal" aria-label="Close"><span aria-hidden="true">×</span></button>
<h4 class="modal-title" id="myModalLabel">Modal title</h4>
</div>
<div class="modal-body">
<?php
echo zen_image($products_image_large, $products_name, LARGE_IMAGE_WIDTH, LARGE_IMAGE_HEIGHT);
?>
</div>
<div class="modal-footer">
<button type="button" class="btn btn-default" data-dismiss="modal">Close</button>
</div>
</div>
</div>
</div>
</div>