Social Previews for Public Bookmarks
When you share a public bookmark, you'll have the option to add a few attributes that help create a preview of your share when it's shared on social media (or posted into Slack). These attributes are completely optional and leaving them blank won't stop your share from working.
Here's where these fields show up:
The image shown above is the default that will be shown if you don't specify an image. That screenshot is taken from Twitter's card validator: a useful tool for previewing how your bookmark's URL will look. You can also use similar tools from Facebook and LinkedIn.
One other suggestion: for best results, your image should be rectangular, with a 16:9 aspect ratio, and be between 1200 and 2000 px wide.