When you create a theme, a preview image will be generated which will appear in your Theme Library.

If you'd like to update that image or the theme's title, you can do so from your theme by following the below steps:

1. Open your Theme.
2. Click into Theme Tab in the top right-hand corner.
3. Click the gear icon next to "Theme." 
4. Click on the current preview image or update the title here.
5. Upload your new preview image.
6. Click "Update Theme."

Now, you're all set! This will be the new preview image for your theme.