The following guide explains how to add a cover picture to your eBooks: https://www.helpndoc.com/step-by-step-guides/how-to-add-a-cover-image-to-epub-and-kindle-ebooks/
The steps involved are the following ones:
- Add a cover page picture to the library using the "Home" ribbon tab, then "Add Item" from the "Library group", then "Add picture"
- Click the top part of the "Generate help" button in the "Home" ribbon tab
- Select your eBook build on the left
- Click "Customize" on the right if the "Template settings" tab is not visible
- Select the previously added "Cover picture" in the "Template settings"
- Generate your eBook again