Customize the image shown when you share a Link Post to LinkedIn or Twitter.
To add your own image to a URL when sharing to Twitter and LinkedIn, go to the "Internal Post" section, add in your URL, add to your group and internal commentary, and then click Upload to add your own photo.

Once you have posted this to a group with your new image, users will then be able to click Share to share the post to their networks.

Below is what the article will look like when a user shares to LinkedIn:

Below is what the article will look like when a user shares to Twitter.
🔷 NOTE: When sharing a custom image to Twitter, Twitter's API changes the link post to an image and wraps the URL in a bit.ly link in the post's content.

FAQs
Can I see what the featured image for Twitter or LinkedIn would be before posting the URL so I can determine if I want to use a custom image?
Yes! Twitter’s Card Validator and LinkedIn’s Post Inspector will provide you with a preview card before publishing it to your profile. Simply insert a URL into the tool and submit it to see what the featured image and description would be for that URL.
Can I share a custom image with a link post when sharing to Facebook?
Due to Facebook's API limitations, adding custom images will not work. When sharing to Facebook only the link will be passed along.
I'm trying to share a Link post and added a custom image but it's not sharing when I post to my network. What could be happening?
Great question! A custom image will replace a single URL's shared image but is overwritten if a second URL is shared in the Internal Commentary. If you are using a bit.ly link or sharing a second link in the share copy, that second link's share image will overwrite a custom image.