To add an image to the theme preview in WordPress, you can use the screenshot file:
1. Create an image that you want to use as the theme preview. The image should be 880 pixels wide and 660 pixels tall.
2. Save the image as a PNG file and name it screenshot.png.
3. Place the screenshot.png file in the root directory of your theme.
4. If you are uploading your theme to the WordPress theme repository, the theme preview image will be automatically generated from your screenshot.png file.
If you are distributing your theme through other means, you may need to specify the theme preview image in the style.css file. To do this, add a Screenshot header to the top of the style.css file and specify the path to the screenshot.png file as the value:
/* Screenshot: https://example.com/wp-content/themes/your-theme/screenshot.png */
I hope this helps!