Jamieson, Fausset and Brown on Deuteronomy 16:22

22. Neither shalt thou set thee up any image--erroneously rendered so for "pillar"; pillars of various kinds, and materials of wood or stone were erected in the neighborhood of altars. Sometimes they were conical or oblong, at other times they served as pedestals for the statues of idols. A superstitious reverence was attached to them, and hence they were forbidden.