Update: Aug. 9, 2017
Create a Theme Screenshot
Once your theme is complete, you need to create a screenshot of it to display in the Theme chooser, shown when a user creates a site or decides to change their theme.
The screenshot must follow these requirements:
- Saved as screenshot.jpg to the root of the theme's directory
- Measure 1590px wide, 894px wide, with a DPI of 480
As long as the screenshot meets these requirements, it will automatically display in the chooser.