How to create Drupal theme screenshot
By default, Drupal will look for the file named screenshot.png in the root of the theme folder and use that as the theme image on the Appearance page.
You do not need to define it anywhere. just create and place screenshot.png in root of the theme folder.
Recommended size of screenshot
The recommended size of screenshot.png image is 480px X 360px
How to change default screenshot name and location
if you do not want to use default name and location of the screenshot, you can define it in the theme-name.info.yml file.