Hi,
Ok, I've looked into it.
The issue is with your script code.
If you look at the javascript it uses, you can see that it basically adds the URL of the image in the img tag with the id expandedImg.
However, if you look at the HTML of the page, you can see that you have twice the gallery on the page with twice the img tag with the id expandedImg:
monosnap.com/file/hXFJ2q0F2YDwImnZk12PBjkLfomF95
Now, this is not valid HTML and cannot work, even in a joomla article. If you add two galleries with your code in a Joomla article, you'll see that it doesn't work properly either.
Please understand that we're not supposed to help with debugging your customizations...