By adding cover_image into the front matter. Click on "Help" to the left of "Markdown" on top when you are writing a post for more info.

 

In case @maestromac 's answer is not clear enough, the top of your post would look like this at the beginning:

---
title: 
published: false
description: 
tags: 
---

And to add the cover_image, you will have to do it manually:

---
title: 
published: false
description: 
tags: 
cover_image: <IMAGE-URL>
---

And that image URL should be ending on a image file extension like .jpg or .png.

Classic DEV Post from Apr 8

My Web Accessibility Testing Process

It sounded like people were curious about my web accessibility testing approach! So I’m going to walk you through my thought process, what I test for, how early I check, and how I test.

I'm the most curious guy I know.