Share images with your contacts and exchange documents through Private Messages nobody else but you and your contacts can read !
A Private Message is like an e-mail: it is made of a subject, some text and attachments.
Privates Messages are sent as attachments through a messaging application, e.g Google Mail.
But before a Private Message is sent, it is encrypted by using an image from your photo gallery: the contact receiving the message must have the same image in order to decrypt the message.