Holy icons have been used in churches, and the faithful have worshiped them as early as Apostolic times. Very many recount this. Together with them, the Seventh Ecumenical Council also put to shame every heretical blasphemy. This Council explains in a most clear manner how one ought to worship the holy icons, and it gives over to excommunication those who render worship to icons as to God, and those who call the Orthodox, who worship icons, idolators. Together with them, we also give over to excommunication those who render either to a saint, or to an icon, or to the Cross, or to the relics of the saints, or to the sacred vessels, or to the Gospel, or to anything else whatsoever in heaven, on the earth or in the sea such honor as is proper only to the One God in Trinity ... as well as those who call the worship of icons idolatry and therefore do not worship them, do not honor the Cross or the saints, as the Church has commanded.
John of Damascus
The first - the natural and in every way similar - image of the invisible God is the Son of the Father, Who reveals the Father in Himself. For, No man hath seen God at any time (John 1:18). And, Not that any man hath seen the Father (John 6:46). And that the Son is the image of the Father, the Apostle says: Who is the image of the invisible God (Colossians 1:15), and in the Epistle to the Hebrews: Who being the brightness of his glory, and the express mage of his person (Hebrews 1:3). And that he shows the Father in Himself, the Lord says concerning this: Have / been so long time with you, and yet hast thou not known me, Philip? he that hath seen me hath seen the Father (John 14:9), He says namely after Philip said: Show us the Father, and it sufficeth us (John 14:8). The Son is the natural image of the Father, completely equal, and in all respects like the Father, except that he is begotten and not the Father...
Since some censure us, who worship and venerate the depiction both of the Saviour and of our Lady, and, moreover, of the rest of the saints and servants of Christ, let them hear that from the beginning God made man in His image (Genesis 1:26-27). And so, why do we bow to one another if not because we are created in the image of God. For, as the divinely-speaking [hierarch] Basil [the Great], who is powerful in the interpretation of divine subjects, says, "the honor given to the depiction passes to the Prototype". And the prototype is that whose image is imprinted, that from which the copy is obtained.
I do not worship matter; I worship the Creator of matter, Who for my sake became matter, and took up His abode in matter, and by means of matter accomplished my salvation.
For, the Word became flesh and dwelt among us (John 1:14), and it is clear to everyone that flesh is matter and is created. And so, I honor matter, through which my salvation was accomplished, and I revere it, and I worship it. But I do not honor it as God, but as full of the Divine operation and grace.
Is not the thrice-happy and thrice-blessed wood of the Cross matter? Is not the honored and holy mount of Calvary matter? Is not the life-bearing rock, the holy tomb, the source of our resurrection, matter? Are not the ink and skins, on which the books of the Gospel are written, matter? Is not the Life-creating Table, which furnishes the Bread of Life to us, matter? Are not both gold and silver, from which crosses, and holy discoses and chalices are made, matter? Are not the Body and Blood of my Lord, which are preferred before all this, matter? Either eliminate the veneration and the worship of all this, or permit, in accordance with the tradition of the Church, the worship of icons, which are sanctified by the name of God and God's friends and for this reason are overshadowed by the grace of the Divine Spirit.
St Ignatius (Brianchaninov)
Stand before an icon of the Saviour, as before the Lord Jesus Christ Himself, Who is invisibly omnipresent and through His icon is present in that place where it is located. Stand before an icon of the Mother of God, as before the Most Holy Virgin herself, but preserve thy mind from visualization. There is the greatest difference between being in the Lord's presence and standing before the Lord, and imagining the Lord. The perception of the Lord's presence brings a saving fear to the soul, introduces a saving feeling of reverence into the soul, while imagining the Lord and His saints communicates to the mind materiality, leads it to a false, proud opinion of itself, leads the soul to a false state - the state of self-delusion.
From the Athonite Patericon
A small image, in the presence of the warmth. of faith and reverence, in the presence of purity of mind and chastity of body, in no way differs from a large image; therefore, be attentive, render worthy honor to the saints and to their sacred countenances, whether they be small or large.