List every images in folder

I have in every pages-folder an images-folder where the user can include background-images for an integrated slider.
In my base.html.twig, I wanted to list all the images in the pages folder and tried to to this with this code:

{% for image in %}
   {{ image.url }}
{% endfor %}

I thought it should work (I saw this method in a skeleton from GRAV) but it doesn’t – it just returns nothing but my images-folder exists and contains two images.
My goal are the URLs of these images. What is my mistake?

The images need to be in the same folder as the .md file, not in an another images/ folder. That’s not needed and just breaking how Grav works.