The wooden sign to the left--which adorned a shop in Coconut Grove, Florida, about 15 years ago--is part of the Carl Hammer Gallery's show "The Art of the Sign," opening today. Some of the signs in the show once advertised actual products (bait, refrigerators, fresh fruit); others were made by artists--like Jesse Howard, whose 20-acre Missouri plot was covered with signs--who wanted to express their religious beliefs and other ideas. All of them, says Hammer, wield as much power on the psyche as any TV commercial. The opening reception runs from 5 to 8 tonight at the gallery, 200 W. Superior. Regular gallery hours are 10 to 6 Tuesday through Saturday; 266-8512.