Let's make a simple header that has a background image filling the page, with centered text. Similar to the one here: http://superbe.be/
I like this design because it gives a quick impression of the site - the image placement gives a lot of freedom to set the tone.
I'm using this css to place the image and text: https://www.w3schools.com/howto/howto_css_hero_image.asp
At first, I wanted to create a block, however this requires styling a region.
Add a Cover region to theme.info.yml.
Print Cover region in page.html.twig.
Add image files to theme.
Add css for Cover region background and content.
TODO: Configurable image.
TODO: Discuss other image scaling options.
TODO: Make image responsive.
TODO: Configure image per page.