Photolikr is a concept, available as a web extension for chrome, a windows app, and some client to rate your pictures.
Everytime you rate a picture, you can build your history and a new wall with all the pictures you have liked, or disliked.
We don't ask the email to send you anything, we promise we don't sell your email and actually we expect you fill something as a unique ID.
it will allow you to find back the information in our database.
you can put a unique ID ( as a phone number with international code )
a key phrase or anything unique, but an email is a good solution no ?