Create the image file in PNG Format having the name screenshot.png and save it in themes root folder.
The 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.
If you don’t already have a screenshot of your theme, You can create one easily by following below steps:
- Point your browser to the blog currently using the theme.
- Press the Print Screen button on the keyboard.
- Open a new file in Photoshop.
- Change the size to 300 by 225 (in pixels)
- Press Ctrl + v or Right click > Paste.
- With the Move tool activated, resize the screenshot to in the 600 x 450 space.
- Go to File > Save for Web…
- Save it as a PNG file.
- Name it screenshot.
- Place your image file in the theme’s folder.