I remember one day sitting in a theater and when a person in the movie brought out a religious icon the child in front of me said to his friend "the church is using computer language" not realizing that the church had icons long before there were computers. But if you were to ask the question how far back do icons go in Christianity, the answer would be St. Paul's letter to the Collosians.
In Chapter 1 St. Paul describes Jesus as the image (icon in Greek) of the invisible God,
the firstborn of all creation. For in him were created all things in heaven and on earth, the visible and the invisible,...
In the Old Testament they were forbidden from making any image of God, because God had not yet made himself visible. With the Incarnation, the invisible God reveals himself, true God and true man. The icon (image) is not like a photograph, drawing, or painting. Jesus is the true icon, God made visible, tangible.
It is from St. Paul's use of the word icon that the tradition of iconography developed particularly in the east. The iconographer was more than an artist. Iconographers were men of prayer. They were expected to live a life consonant with their vocation. The process of creating an icon involved not only painting but long periods of prayer. Just as Christ the icon revealed the thereto fore invisible God, so any icon is meant to reveal more than the physical likeness but also reflect the presence of the Holy Spirit that, as St. Paul also tells us,dwells in all the saints. As a matter of fact the physical likeness of an icon to the person (Jesus, Mary, or one of the patriarchs, prophets or saints) is of a secondary importance. The role of the icon is to connect us to the spiritual reality.
Unfortunately many of us have only ever seen the mass produced icons easily available. If you have a chance visit an Orthodox Church and spend some time in prayer mediating before an icon hand painted by a true iconographer. My first such experience was more than 30 years ago in a monastery in Russia, and I will never forget it.