I note this here, because when I was a Lutheran I thought just the same thing: idolatry. I saw an Orthodox Church that had only the Virgin featured - lacking the Christ Child in her lap. This is non-canonical. However, the Theotokos' placement behind the altar, whether in the canonical fashion or the non-canonical, makes a particular confession of faith. Lutheran onlookers (like myself those years ago) shudder and wonder how a huge icon of the Theotokos directly behind the altar, center-stage, could be anything but a confusion of who we are there to worship.
Yet the Orthodox Church places the platytera there to emphasize a particular theological truth - one that I learned from one of my seminary professors as a Lutheran. The Virgin Mary is the fulfillment of the Ark of the Covenant. In seminary it was even pointed out that her movements following the Annunciation follow the likeness of the Ark of the Covenant's movements in the Old Testament.
The Ark of the Covenant was covered with the mercy seat, the Throne of God flanked by cherubs. It was kept in the Most Holy Place in the tabernacle. Upon this mercy seat the blood of atonement was sprinkled once a year by the High Priest for the forgiveness of sins for the people of Israel. The Ark of the Covenant contained within it Aaron's staff which had budded, the jar of manna that came down from heaven, and the tablets of the Ten Commandments.
The Virgin Mary fulfills this Ark. She is the one in whom the Word of God dwelt, in whom the True Manna came down from heaven and remained. She is the one who, though a Virgin, was made to bud with Life Himself, fulfilling Aaron's staff. She is the one upon whose lap the Lord God of Abraham, Isaac, and Jacob sat enthroned for our salvation.
Thus the Orthodox Church rightly confesses her to be Theotokos, and rightly places her holy icon at the center behind the iconostasis. She is the True Ark of the Covenant with its mercy seat, upon whose lap and from whose womb we find the Lord Jesus seated as True Man for our salvation. So the placement of her icon - canonically or uncanonically - confesses the same truth: that God has appeared in the Flesh, and has fulfilled the Old Testament's shadow with the Light of His coming. We enjoy the reality of that coming now, gathered around the Eucharist, around our Lord Jesus who unites all in Himself, with His Father and the Holy Spirit ever reigning. Amen.