Christ Between St. Peter and St. Paul
Catacomb of Saints Marcellinus and Peter on the Via Labicana
This is one of the earliest images of an enthroned Christ. As in others of this type, Saints Peter and Paul stand at his left and right. Peter wears his usual square beard; Paul has a long beard and receding hairline. Both wear white togas.
Christ is portrayed in a dark toga with a rather long beard. Many other paleo-Christian images show him much younger. Left and right of his halo are the alpha and omega of Revelation 1:8 ("I am Alpha and Omega, the beginning and the end, saith the Lord God, who is, and who was, and who is to come, the Almighty"). The two letters seem more crisply defined than the rest of the painting. Perhaps they are a later addition.
Read more about images of St. Peter
Read more about images of St. Paul.
Read more about images of Christ in Majesty.
Source: this page at Wikimedia Commons.