Nicea Council II addressed iconoclasm, the movement against images of Jesus and the saints. In the late eighth century, it was a crisis. Islam, of course, rejected all images of sacred persons from the previous century onward. Christians dealt with the crisis in present-day Turkey under the patronage of the Byzantine Empress.
238. The Second Council of Nicea, “following the divinely inspired teaching of our Holy Fathers and the tradition of the Catholic Church”, vigorously defended the veneration of the images of the Saints: “we order with ever rigor and exactitude that, similar to the depictions of the precious and vivifying Cross of our redemption, the sacred images to be used for veneration, are to be depicted in mosaic or any other suitable material, and exposed in the holy churches of God, on their furnishings, vestments, on their walls, as well as in the homes of the faithful and in the streets, be they images of Our Lord God and Savior Jesus Christ, or of Our Immaculate Lady, the holy Mother of God, or of the Angels, the Saints and the just” (Second Council Of Nicea, Definitio de sacris imaginibus (23 October 787), in DS 600).
Why are images accepted? Is it more than just people longing for images of those they love and regard? The Church has a theological justification, that the Second Person came to Earth.
The Fathers of Nicea see the basis for the use of sacred images in the mystery of the Incarnation of Christ, “the image of the invisible God” (Col 1, 15): “the Incarnation of the Son of God initiated a new ‘economy’ of images”(Catechism 1161).
The Directory on Popular Piety and the Liturgy is online at the Vatican site.