Well, hopefully the iconography that exists in the Roman catecombs should be helpful - it is most certainly pre-4th century, and while simple, was the jumping point for iconography in the Church.
Yes I would also add the "Dura Europos" churches and synagogues to the list.
Dura-Europos synagogue
The world's oldest preserved Jewish synagogue was dated by an Aramaic inscription to 244. It was preserved, ironically, when it had to be infilled with earth to strengthen the city's fortifications against a Sassanian assault in 256. It was uncovered in 1935 by Clark Hopkins, who found that it contains a forecourt and house of assembly with frescoed walls depicting people and animals, and a Torah shrine in the western wall facing Jerusalem
http://en.wikipedia.org/wiki/Dura-EuroposAs well of a mention by Eusebius that there were portraits of the Lord, that were known in his time. (He disapproved of the practice of them).
And besides that "The legend of Addai"
The purely legendary tale of how Abgarus of Edessa and Jesus had corresponded was first recounted in the 4th century by the church historian Eusebius of Caesarea in his Ecclesiastical History. (i.13 and iii.1) and it was retold in elaborated form by Ephrem the Syrian. In the origin of the legend, Eusebius had been shown documents purporting to contain the official correspondence that passed between Abgar and Jesus, and he was well enough convinced by their authenticityto quote them extensively in his ecclesiastical history. By the time the legend had returned to Syria, the purported site of the miraculous image, it had been embroidered into a tissue of miraculous happenings (Bauer 1971, ch. i): the Doctrine of Addai is full of miracles, and anti-semitism in the garbled story of "Protonice" consort of Claudius, searching for the Cross, and Golgotha and the Holy Sepuchre, all of them in possession of the Jews.
http://en.wikipedia.org/wiki/Addai
