PROBLEM Get Simple CMS – Problems with HTML Data Attributes?

[Sveninho]

Hello
I'm working on a new Project using GetSimple CMS again. In this Project I'm using Pixelcoq's parallax.js. to slighly animate the images on scroll. (http://pixelcog.github.io/parallax.js/)

I'm targeting the images via HTML Data Attributes. Using GetSimple CMS I'm targeting the image folder as usual:

<section class="parallax-window" data-parallax="scroll" data-position="top" data-bleed="100" data-speed="0.3" data-image-src="<?php get_theme_url(); ?>/img/image.jpg" data-natural-width="1920" data-natural-height="1281">

           </section>

For some reason the images do not display after uploading to the server. I tried to fix it with targeting the images using JS but that doesn't work either ...

Ideas, solutions how to fix this are more than welcome.

Sven