When changing to a new theme, you need to regenerate your thumbnails (featured images). You can do this by using the Regenerate Thumbnails plugin which is included in your theme’s recommended plugins.
- Install and activate the Regenerate Thumbnails plugin.
- From your dashboard, go to Tools > Regenerate Thumbnails.
- Click the Regenerate Thumbnails button.
If this doesn’t help, your images might be too small. Try uploading them at a larger size if possible.