Change header image on the home page

Sorry for this basic question, but I didn't find the answer in the Forum.
I would like to change the header image in blog theme or one-page skeleton theme.
The initial image format seems to be embedded in the css style and the image itself is stored in the cache (as far as I understood).
Is there a possibility to add it in the custom.css file ?

Thank you

Actually this is pretty simple. First the image is cached, but it's based on the image in the home page's folder. So for both the one-page and the blog skeleton, it's the only image in the user/pages/01.home/ folder.

The colorization is happening dynamically and is in the Twig template.

For the blog skeleton, this is in user/themes/antimatter/templates/blog.html.twig:

{% set blog_image = page.media.images|first.grayscale().contrast(20).brightness(-100).colo rize(-35,81,122) %}

Similarly for the one-page skeleton it's in: user/themes/antimatter/templates/modular/showcase.html.twig

{% set showcase_image = page.media.images|first.grayscale().contrast(20).brightness(-125).colo rize(-35,81,122) %}

Notice a couple of things. First the twig template matches the filename of the page. So 'blog.mduses theblog.html.twigtemplate, and the showcase modular page (part of the one-page modular homepage) is controlled by 'showcase.md and therefore uses modular/showcase.html.twig

Also note, that these twig commands are dynamically adjusting contr ast/brightness/color, but you can remove those if required, or change them. More details here on the media page of the docs. Also It's grabbing the first image it finds in the directory. You could grab a specific image by putting page.media['myfile.jpg'].