I have set up a child theme of the twentyten theme. In the themes section that under the appearance, I can only see the child theme's title, author name, and a grey box above it. How can I add an icon/image to be displayed in the grey box?
Leave a file with the name
screenshot.png inside the root of your child theme folder and it works.
The recommended image size is 600x450. The screenshot will only be shown as 300x225, but the double-sized image allows for high-resolution viewing on HiDPI displays.
- see on Codex
As @bueltge suggested simply create a
screenshot.png in your child theme folder.
You can get more information from the WordPress Codex here.