If you visualize someone's picture strongly you 'call' that person. 'Dead' or alive. It doesn't matter whether it is pixels or paper. What matters is the forming and holding of the image in your mind. It is also helpful to recall the sound of their voice. Say their name. Theres no good way to explain it. I wouldn't believe it myself if I did not have certain experiences in yoga. The experiences show you it IS possible so you know your not wasting your time. The practice of concentration, holding your mind to one thing or idea/your attention span fixated on one thing however takes great skill as well as manipulating the third eye.
Its sort of like it is much easier to understand what other people are saying in another language. But it is much more difficult to speak yourself in that language because you lack confidence. In the same way, not having psychic experiences a person only half believes in what they are doing by visualizing someone. This confidence is a great boost. Then, you get a knack for it by practice which again is a big help. Many things influence your success that are outside of your control. If the person is beginning to go to sleep you have the best chance of success. In waking consciousness, your inner voice is basically shouting so loud you can not perceive anything else.