PALM TREES AND swimming pools. Sunsets and surfers. Beautiful bodies and no brains. Psychic nutritionists and pet psychiatrists. Hollywood wives and Valley girls. These are the L.A. cliches, the images and attitudes hauled out time and again to describe Southern California and the people who live here. But a city on the way to becoming the largest in the country and the American capital of the Pacific Rim deserves a new image. So, six Southern Celifornia advertising agencies were asked to design a conceptual poster that would update the L.A. image for 1989. "How would you sell the new L.A. to the rest of America?" we asked. What you see is what they begat.