To Vaux and Olmsted the entire park, with its crags and cascades and meadows, was a single work of art, minutely planned and painstakingly executed. So it is particularly fitting that a nonprofit group called the Central Park Conservancy chose to raise funds by mounting an exhibition of paintings of the park from its beginnings to the present day. Most of the pictures in this portfolio have been drawn from that show, which was held at New York’s Hirschl & Adler Galleries last spring.