Yes, you can easily delete any transformation photos uploaded to the app. Go to the 'Profile' section of your app, tap on 'Transformation photos' and select the relevant date(s) in the history list. Click 'delete' and go through the confirmation page. The photos will be permanently deleted, so you won't be able to access these via the app any longer.
If you want to replace the photos with new ones, just click ‘edit’ and either take a photo or upload an existing file from your photo library.