This is the Virgin with Child in the apse. Its called the Theotokos mosaic, and dates from the second halve of the 9th century. Theotokos: "God-bearer". Enc. Britt.:in Eastern Orthodoxy, the designation of the Virgin Mary as mother of God. The term has had great historical importance because the Nestorians, who stressed the independence of the divine and human natures in Christ, opposed its use, on the ground that it compromised the human nature of Christ, and held that the more accurate and proper term for Mary was Christotokos (“Christ-Bearer”). The Council of Ephesus (431), basing its arguments on the unity of the person of Christ, anathematized all who denied that Christ was truly divine, and asserted that Mary was truly the mother of God.)