Good Shepherd fresco from the Catacombs of San Callisto
Bearded Christ, from catacombs of Commodilla
Oldest known image of the St. Paul (interesting to compare this with El Greco's painting.)
Below is "Fractio Panis" on the face of the arch immediately over the altar tomb, upon which the sacrament of the Eucharist was performed (Catacomb of Priscilla):
"The Breaking of Bread was not just the opening gesture of the agape as such, but was surrounded by a complex liturgy: there were psalms, readings from the prophets, homily of the celebrant, etc."
Interesting Wiki description of the painting.
From the site above a section on Spirituality of the Catacombs. The opening graphs of this section:
An unknown believer in early times, while wandering through the vast Callixtian complex, all of a sudden thought he had entered the mystical Jerusalem, a city purpled by the blood of martyrs and resplendent with their glory. As he went out he carved on the plastered wall near the crypt of the Popes a statement we can still read to-day: "Gerusale, civitas et ornamentum martyru(m) D(e)i ..." - "Jerusalem, city and ornament of God's martyrs ...". To-day's pilgrims can likewise grasp the innermost secret of the spirituality of those martyred Popes, of those Virgins and of the numberless host of humble Christians.The inscriptions and paintings, which have survived the many ravages and pillages, reveal at least in part such a secret and still repeat the words engraved in an ancient epitaph "Tàuta o bìos" - "Such is our life".Such an incredible wealth of information about the very early Church: the sacraments, liturgy, symbology, the earliest Christian art..The spirituality of the catacombs is the same as that of the primitive Church. Nourished on the marrow of Scriptures, simple, yet powerful, it is the sister of the most ancient liturgies; so that the visitors to the catacombs can draw from the very sources of Christian spirituality.