How to remove the backed up images on WordPress?
To remove the original files saved by ShortPixel Image Optimizer upon optimizing the images of your website, go to Settings > ShortPixel > Statistics, and at the bottom of that tab you'll see an "Empty backups" button. Use it to have all the saved original files removed.
Another way to do this is to just use an FTP client (like Filezilla) or your hosting's file manager: go to the path
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 to a local storage (e.g. your computer). If you ever need to restore the backup in the future, you can just upload the same files and folders back to your website, on the same path, and ShortPixel will be able to use them again.