I noticed Tumblr seems to have added a feature that captures a story image (or link preview) when a link post is made.  This is not unlike how Facebook and other social media sites capture an image for a URL when posted.  How can I get the image to display on a link post in a custom theme?  There doesn't seem to be any documentation and I cannot find an example from an existing theme.
Images from Tumblr Link Posts
tumblr;links;tumblr themes