WordPress looks for a .png image file named, screenshot, in your theme’s folder. If you don’t already have a screenshot of your theme, here’s how:
- 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 1200 by 900 (in pixels)
- Press Ctrl + v or Right click > Paste.
- With the Move tool activated, resize the screenshot to in the 300 x 225 space.
- Go to File > Save for Web…
- Save it as a PNG file, instead of GIF or JPG.
- Name it screenshot.
- Place your image file in the theme’s folder.