Unless you create duplicates, only one copy of each photo exists is your Photo Library. Echo Photo can be “assigned” to one or more Albums - each album is effectively a subset of your full Photo Library.
Deleting a photo from an Album just removes the “reference” that assigns a photo to the album - and does not, necessarily, delete a photo from your Library. As such, removing a photo from an album frees no space; deletion of a photo from the Photo Library moves the photo to the Recently Deleted folder - where it will remain for 30-days prior to automatic deletion.
Deletion from your Photo Library - and then from your Recently Deleted folder, will fully delete the photo. If using Photos in iCloud, the photo will be removed from your iPad and iCloud - releasing space in both locations.
As for privacy, unless you have shared your photos using iCloud, or allow other Users access to your devices, nobody other than you can view your photos.