St. Demetrios mosaic

A pre-Iconoclastic depiction of St. Demetrios at the Aghios Demetrios Basilica. Note the dark blue tablion on his chest.

St. Demetrius was initially depicted in icons and mosaics as a young man in patterned robes with the distinctive tablion of the senatorial class across his chest. Miraculous military interventions were attributed to him during several attacks on Thessaloniki, and he gradually became thought of as a soldier: a Constantinopolitan ivory of the late 10th century shows him as an infantry soldier (Metropolitan Museum of Art). But an icon of the late 11th century in Sinai shows him as before, still a civilian.
Another Sinai icon, of the Crusader period and painted by a French artist working in the Holy Land in the second half of the 12th century, shows what then became the most common depiction. Demetrius, bearded, rather older, and on a dark horse, rides together with St George, unbearded and on a white horse. Both are dressed as cavalrymen. Also, while St. George is often shown spearing a dragon, St. Demetrius is depicted spearing the gladiator Lyaeos, who according to legend was responsible for killing many Christians. In traditional hagiography, Demetrius did not directly kill Lyaeos, but rather through his prayers the gladiator was defeated.
A modern Greek iconographic convention depicts Demetrius with the White Tower of Thessaloniki in the background. The anachronistic White Tower acts as a symbolic depiction of the city of Thessaloniki, despite having been built in the 16th century, centuries after his life, and the exact architecture of the older tower that stood at the same site in earlier times is unknown.

http://en.wikipedia.org/wiki/Saint_Demetrius_of_Thessaloniki