A featured image should be added to each post. The featured image displays as a thumbnail image with the post. If there is no featured image, post summaries will display a blank box where the thumbnail should be.
However the WordPress theme that we are using also adds the featured image as a header to the post, with an option to disable this by ticking Disable Featured Image for this Post (in the Post Options section). More accurately, the text for this tick box should read Disable Featured Image as post header (for this Post).
If the featured image has a small image size, it will be expanded when used as a header and the image will appear pixelated and unclear. In this case, the recommended method is to
- Add a featured image for the thumbnail
- Add a suitably large header image (at least 800 pixels wide) using the Cover type block in the new editor (in Common Blocks). This block type also allows the addition of a title.
For more information, see this article.