"In what way is this functionally different to uploading the photo to Farcebook and letting them hash it?"
You can't reconstruct the photo from the hash, so if FB only get the hash, they can't "lose" the photo.
"In what way is this functionally different to uploading the photo to Farcebook and letting them hash it?"
You can't reconstruct the photo from the hash, so if FB only get the hash, they can't "lose" the photo.