Mosaics in ancient Rome are now thought to have had the purpose of driving away the evil eye and envious people. According to Professor of ancient history Luz Neira who coordinates a team of researchers into ancient ritualist practices of religion and magic in ancient times.
The wealthy in Rome would commission these works of art for their homes to ward off evil spirits. for instance an eye pieced by a lance and surrounded by animals. Interestingly they would also put up representations of the phallus and other objects to ward off the spells of the envious. One such depiction in a mosaic on the island of Cephalonia is an envious person represented by someone writhing and strangling themselves because of the envy produced by a house they were viewing.
The mosaic posted here is the sacrifice scene St. Roman in Gal.