Creating a child theme on WordPress only requires one folder and one file, that’s for sure very sweet, but after you create it, if you go to your Theme selection tab on Appearance/Themes/ you will see that there’s no thumbnail for your new child theme.
That could be considered a really small and unimportant detail, but if you want to be professional with your work, this is one of those details you should take care of at a very early stage of your theme development.
First thing to do is to create a small 300 x 225 image. Size could vary, but if you want to keep yourself into the standards, this is the fixed size of every theme thumbnail on wordpress.
After you create your image, save it as PNG file and put it on the theme’s folder, just beside style.css
Once there, WordPress automatically will make the screenshot to appear on Appearance/Themes/ and your theme will have more presence on the theme’s list.