Wordpress - How to set thumbnail image for a (child) theme

Create image file in PNG Format having name screenshot.png and save it in themes root folder.The recommended image size is 880x660. Even recommended image size is 880x660 though it will only be shown as 387x290 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.

What Vinod Dalvi said, except:

Make sure the file is named using all lower case:

• screenshot.png

or

• screenshot.jpg.

screenshot.PNG won't work!