The giant early 17th-century mosaic inside of the cupola has as its main theme Christ the Redeemer and (mostly) his disciples. They are accompanied by numerous angels. On the medallions are the four evangelists while God the Father is depicted high up in the lantern.
Note the gallery railing at the bottom of the photo — it runs around the whole drum just below the dome and allows for wonderful views into the church and of course the dome mosaic too.