Set this once. Everything else on this page — your categories, your picture library, your existing posts — is read from the website through these two settings.
The cover is what shows at the top of the post, on every article card, and in the preview when the link is shared. Landscape works best — around 1200 × 675. If you skip it, the post falls back to the emoji below.
Pick up to three of your own posts. Leave them all on None and the site shows the three newest posts in the same category instead.