How to remove the backed up images in WordPress?
To remove the original files saved by ShortPixel upon optimizing your website's images to to
Settings/ShortPixel and then to Statistics tab and at the bottom of that tab you'll see an "Empty backups" button. Use it to have all the saved original removed.
Another way to do this is just to go (via FTP for example) to /wp-content/uploads/ShortpixelBackups/ and remove all the files from there.
Please note that you can also choose to move all the files from that folder on a local storage (e.g. your computer). If you ever need to restore the backup in the future you can just upload the files back to your website, ShortPixel will still be able to use them.