Hubert Robert painted an imaginary Louvre in ruins. For a taste of transience on Wilshire Boulevard, check out the increasingly ruinous banners on Renzo Piano's Broad Contemporary Art Museum. After two years, the John Baldessari-designed signage is beginning to sag from its supports and has taken on the sickly blue cast of an ad in a luncheonette window. In days past, there was talk of ongoing artists' commissions for new banners. The recession intervened, or maybe they want to keep the banners in place for the upcoming John Baldessari show. Either way, the images won't look any fresher with another summer's worth of UV.