Your original images are saved in a backup folder located on your server. The path is
/wp-content/uploads/ShortpixelBackups/. To access that backup and download your original images, you can use an FTP client like Filezilla (please contact your hosting provider to help you with this), or access the image directly from your browser. For example, if your optimized image is located on
https://example.com/wp-content/uploads/2020/06/image.jpg
the backup will be on
https://example.com/wp-content/uploads/ShortpixelBackups/2020/06/image.jpg
You can
remove the backup folder at any moment but we highly recommend keeping a local copy, in case you ever need to restore your images.