Hi,
I have created a portfolio (named it differently) by creating a new post type.
Under this portfolio I wanted different categories. So added a new "portfolio" and named it "atelier" . Here I inserted pictures and a thumbnail and a featured image.
But when I view the portfolio page I see text "atelier" but no thumbnail or grid of pictures the way it is shown in your demo.

Do you know how I can fix this?

The website is not live yet. So I cannot send you an url.