I posted a link to one of the A&S pages in a tweet, but the image is not showing in the feed.
In order for Twitter to properly generate an image with the link, you must add an image to the Thumbnail. You can do that in Page edit mode and then select Open Properties under Page Information (in the Action Bar on top). Once the Properties panel is opened, simply click on the Thumbnail tab and drag&drop an image from the Assets side panel. Make sure you save your edit and publish the page.
A description can also be added to the tweet by adding a description to the page in the Properties panel, under the Basic tab. Please note that the Description field is also displayed under the main header on most templates