The Crucifixion

Icon from the monastery of St. Catherine, Sinai, mid-8th century
Tempera on panels

The Greek word beneath Christ's right arm can be transliterated as GESTAS.  The partial word beneath his left arm begins with the Greek equivalent of DEM -- presumably a variant of "Dismas."  The inscriptions below those two words identify Mary and John respectively.

More of the Crucifixion

Source: Merback, 'The Thief, The Cross, and the Wheel'