First half of 5th century
Mausoleum of Galla Placidia, Ravenna, Italy
As in most early images, the saint wears a shorter form of the Roman toga over a white ankle-length tunic or alb. As in almost all images, he is balding from the front. But this mosaic does not give him the usual pointy beard.
The portrait is found in one of the apses:
The first image in full resolution
More of St. Paul
Photographed at the site by Richard Stracke, shared under Attribution-NonCommercial-ShareAlike license.