Add a Screenshot to your Drupal 8 Theme

To add a Screenshot to your Drupal 8 theme, you need to follow the following steps

Add some data and images to your site so the theme looks nice

Take a screenshot of your site using Firefox screenshot tool

Crop the Image and Download it

Edit the Screenshot and resize it to a height of 438px while maintaining the height to width ratio

Save the image as screenshot.png and upload it to the theme folder

Then navigate to Yoursitename.com//admin/appearance and you will see the theme showing with the screenshot

Thank you and looking forward to help you do more with Drupal