In the St. Isaac Cathedral, the Iconostasis (icon screen) that separates the nave and the sanctuary is elaborately decorated.
Per Orthodox Church liturgical guideline, the holy doors at the center depicts the Annunciation.
On the left is an icon of Theotokos (mother of God).
On the right is an icon of Pantocrator (the Lord as ruler).
Above the holy doors, the depiction of the Last Supper is out of this photo.
Only priests performing liturgical duty may enter the holy doors to approach the altar.