How to set thumbnail image for a (child) theme

I’ve created a basic child image for twentytwelve, and it works as expected. The only issue is that it does not have a thumbnail image in the themes page:

How do I set a thumbnail image for my child theme?

Create image file in PNG Format having name screenshot.png and save it in themes root folder.The recommended image size is 880×660. Even recommended image size is 880×660 though it will only be shown as 387×290 but the double-sized image allows for high-resolution viewing on HiDPI displays.

For more information see this page.

If you don’t already have a screenshot of your theme, You can create one easily by following below steps:

1. Point your browser to the blog currently using the theme.
2. Press the Print Screen button on the keyboard.
3. Open a new file in Photoshop.
4. Change the size to 300 by 225 (in pixels)
5. Press Ctrl + v or Right click > Paste.
6. With the Move tool activated, resize the screenshot to in the 600 x
   450 space.
7. Go to File > Save for Web…
8. Save it as a PNG file.
9. Name it screenshot such as screenshot.png
10. Place your image file in the theme’s folder.