How to put a banner picture on the antimatter home page (like on getgrav.org)?

How to put a banner picture on the antimatter home page (like on getgrav.org) ?

Your best option is to download the One-Page Site Skeleton package in the downloads.

This is a ready-made example of a a modular layout that is actually based on the getgrav.org homepage. You will see how the header is actually a showcase modular section in a single modular page. The image can be replaced with whatever you want, and the colorizing is dynamically done in the /user/themes/antimatter/templates/modular/showcase.html.twig file so you can change that as you like.

The other option is to use the technique used in the Blog Skeleton and that is similar but defined in the antimatter/templates/blog.html.twig file. With a body class being set in the headers of the markdown so that the CSS can add the appropriate padding.

Thanks !
I see that the sass is compiled in css. What compiler do you use ?

You can really use any compiler, but we generally use the ruby compiler installed via gem. http://sass-lang.com/install

There is a sass.sh script file in the antimatter directory that will watch and compile to the correct place once you have sass installed.

Well on my installation i replaced the showdown.jpg with another one. And now getting an error image. Have deleted the /images and /cache folder but still not the image i want to load...

rather than manually deleting those folders, can you run the CLI command: bin/grav clear-cache ??