I always ASK all my clients if they would like to have a before and after picture taken, most say yes. I then take a photo every two stone and let them see a comparison of themselves. This can be really helpful as sometimes it can take a while for your brain to catch up with just how much you've lost. Even on the heaviest of clients two stone is usually quite a difference. I put the photos side by side using photoshop and give the client a copy of them.
I will only ever show clients photos of other clients when I've had permission to do so.
The clients who have finished and do not want me to show them to other people are given a copy and then I delete them.
The clients who did not finish, I keep them in a separate folder secured by a password. All photos are saved under initials and date, so people cannot be identified from them by their name.
This isn't a service we need to offer as Consultants, its just an additional service that I offer to my clients and I'm pretty sure that the ones who take me up on it are really grateful for it. I never push anyone into it, if they don't want a pic then so be it.