West dome, St. Mark's Basilica
This image closely follows the narrative in Acts 2:1-13. The Holy Spirit descends on each of the twelve Apostles in tongues of fire, leading them to preach to the crowd. The Spirit is symbolized in the center as a dove sitting on a book, which lies on a blue sheet inscribed with gold IX monograms. The sheet covers a throne, and the throne is centered in a multicolored disc that most likely represents the cosmos.
The words surrounding the disc narrate the event: spiritus in flammis super hos distillat ut amnis corda replens munit et amoris nexibus unit hinc varie gentes miracula conspicientes fiunt credentes vim lingue percipientes, The Spirit pours over them, strengthening them by filling their hearts of each and uniting them by bonds of love. Then the nations become believers, seeing the miracle of the various languages."
The Apostles sit on thrones, most likely a reference to Matthew 19:28, "And Jesus said to them: Amen, I say to you, that you, who have followed me, in the regeneration, when the Son of man shall sit on the seat of his majesty, you also shall sit on twelve seats judging the twelve tribes of Israel" (c.f. Luke 22:30).
Each Apostle holds a book or scroll. The two at the "bottom" of the photograph are Peter and Paul, the former identified by his short, square beard and the latter by his high forehead and pointy beard. The Apostle to the right of Paul looks younger than the others and is probably John. Andrew, in the center right of the photo, can be identified by his wild hair.
The small figures in the outermost ring are the people in the crowd who gathered to hear the Apostles preach. "Jews of every nation," they are identified in the inscriptions with the list of origins in 2:9-11: Parthians, Medes, etc.
Read more about Pentecost.
Source: Wikimedia Commons.