Aside from using featured images,
<img/> HTML tags in the content, etc., WPSSO can also include attached
images in the Open Graph and Twitter Card meta tags. Attached images
have a link to the Post or Page, but are not displayed by default
(unless they are also included in the content). To attach an image to a
Post or Page, go to the Media -> Library and upload a new image, or
use one that is already listed with ‘(Unattached)’ under the ‘Uploaded
to’ column, and click on its ‘Attach’ link.