Creating a screenshot for a child theme
For your child theme, you probably want to create a screenshot that is immediately recognisable as belonging to the specific website it was created for, rather than just duplicating the parent theme image.
The recommended screenshot size is now 1200 x 900 pixels. Actually, if it is smaller that is OK, just make sure to stick to the 4:3 ratio.
Save the image as “screenshot.png” in the child theme folder, next to your style.css and functions.php.
Note: on the page which shows all your themes, your image will be cut off, so the visible area will be 1200 x 800 pixels. Make sure you keep any branding out of the bottom 100 pixels for clarity.