How can I add a 'cover image' to collection items in the admin panel?

Question

Using Decap CMS (formerly Netlify CMS) -- In the admin dashboard, how can I add a cover image/thumbnail to a collection item using the _data directory? On the demo site from their documentation:

https://demo.decapcms.org/#/collections/posts

When changing the view from 'list' to 'icon' and scrolling to the bottom, there are items in the collection that have a cover image/thumbnail. See screenshot for reference.

admin dashboard showing collection items with a configurable thumbnail image

In my config.yml I've tried multiple times and different ways similar to the pattern on their github repo/sample code:

https://github.com/decaporg/decap-cms/blob/main/dev-test/config.yml

But haven't had any success.

Thanks for your time.

Within the config.yml I've tried adding two different code snippets based on the code sample (github repo) from Decap CMS's documentation

// config.yml

collections:
  - name: "blog"
    label: "Blog"
    folder: "src/posts/blog"
    create: true
    editor:
      preview: false
    fields:
      - { label: "Title", name: "title", widget: "string" }
      - { label: "Description", name: "description", widget: "text" }
      - { label: "Author", name: "author", widget: "string" }
      - { label: "Publish Date", name: "date", widget: "datetime" }
      - { label: "Body", name: "body", widget: "markdown" }
      - label: "Cover Image" # Only works for 'post' folder, NOT _data
        name: "cover_image"
        widget: "image"
        required: false
      - widget: select
        label: "tags"
        name: "tags"
        multiple: true
        create: true
        options: ["popular", "general", "physical", "emotion", "finance"]