The trend toward greater visibility of Mexican art traces to the increasing number of Hispanics in United States cities, growing appreciation for indigenous cultures, and the publicity surrounding the 500th anniversary of Columbus's landing.Art-auction companies hope Latin America will provide the next art boom, since the market for Impressionist works has tapered off. Two years ago, no painting by a Latin American artist had broken the million-dollar mark; now, according to the Washington Post, there are five. Recently, the painting "Vendedora de Flores," by Mexican artist and muralist Diego Rivera, sold for $2.9 million. Frida Kahlo's haunting work, "Self-Portrait With Loose Hair," brought $1.65 million at Christie's last spring. In connection with the exhibit "Mexico: Splendors of Thirty Centuries," Los Angeles is sponsoring a city-wide cultural festival. For updated information on the festival, call 213-688-ARTS. A sampling of Mexican art exhibitions in other areas includes: * Leonora Carrington: The Mexican Years, 1943-1985, through March 1. The Mexican Museum, San Francisco.r New Traditions: Thirteen Hispanic Photographers, Staller Center for the Arts, State University of New York at Stony Brook, through Dec. 18.