I’d prefer to use similar methods seen in the blog and one-page Antimatter skeletons, but I can’t get it work on a standard page (jpg in 01.home etc.).
I think there is a new feature allowing this to be done via the Admin panel and the latest version of the Antimatter theme using blueprints, but when I diff the code (1.7.2 vs. 1.7.3) it looks like the change is in blog_item.html.twig (I assume blog_item.html.twig is not used by a standard page).
So what is the cleanest way to have the same image as a header/banner on a site comprised of standard pages?
If you want to have the same image in a header banner, I suggest modifying the (or creating a new) page template. The default.html.twig is the standard one, but you could just copy that and create a new banner-page.html.twig or something and add the image logic to that page. Then just choose that template for all pages that you want to have the banner displayed on.