Theme: Bartik Subtheme, custom made. Changes in custom.css.
Admin Theme: Core Seven. core/themes/seven/css/components/form.css:214 change: font-size: 1em > 1.2em, in core/themes/bartik/css/base/elements.css: change fon-size 87.5% to 108%; because the admin font in the ck-editor iframes still comes from the original Bartik Theme, not the Bartik Subtheme.
Banners are 1280 x 440 px, 75-150 KB.
We use core Media and core Media Library - not the non-core Entity Browser and related.
We upload new images with the Media content type. Content > Add content > Media > Create new media > Media add > Image. This is also where the image's taxonomy terms are entered. At the moment we do not have and do not need a separate content type for images,