Set header image on antimatter theme

Can someone explain me how to add a header image to the antimatter theme. Like it's done in all of the preview images.

I smiply used HTML:

<div align="center">
<img src="path-to-your-image.jpg"></img>
</div>

Antimatter automatically resizes it to fit on any screen-size.

If you download the blog-skeleton package you can get a working example of the demo: http://demo.getgrav.org/blog-skeleton/ You can see how the header image for each post is pulled out automatically and added to the top of the post by looking at the partials/blog_item.html.twig file. Specifically this bit:

{% if big_header %}
    {{ page.media.images|first.cropResize(900,600).html }}
{% else %}
    {{ page.media.images|first.cropZoom(900,300).html }}
{% endif %}