An alternative is
Colorbox, which ships with excellent styles and is also relatively lightweight.
Just include Colorbox and JQuery (which you already have):
<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.11.2/jquery.min.js"></script>
<script src="assets/js/jquery.colorbox-min.js"></script>
Then load it:
// Where .my-selector is the selector of your gallery items ( thumbTpl )
$(".my-selector").colorbox();
Don' forget the stylesheet:
<link href="assets/css/colorbox.min.css" rel="stylesheet">
As long as you add the .my-selector class to your gallery thumbTpl, Colorbox should work. Here's an example:
<figure class="[[+cls]]">
<a href="[[+linkToImage:if=`[[+linkToImage]]`:is=`1`:then=`[[+image_absolute]]`:else=`[[~[[*id]]?
&[[+imageGetParam]]=`[[+id]]`
&[[+albumRequestVar]]=`[[+album]]`
&[[+tagRequestVar]]=`[[+tag]]` ]]`]]" class="my-selector">
<img class="img-responsive" src="[[+thumbnail]]" alt="[[+description]]" />
</a>
</figure>
I hope this helps.
PS: The Kontakt link of your website (footer) is broken. You have [[~36] instead of [[~36]]