The key image for an entry is the image that appears on any card view of the entry, like on the front page of the site or on a circle page.
To remove the key image on your entry, you must first be editing the entry.
On the Entry Edit screen, above the editing toolbar you will see a “Remove Featured Image” button:
To remove the key image, click the Remove Featured Image button. Once the image is removed, the button will change to “Set Featured Image”.
If you want, you can add another key image.