Sierra Nevada from Lone Pine, California (Winter Sunrise)Ansel Adams, 1944Est: $300,000-$500,000David Hockney once said he couldn’t look at any photograph for more than 30 seconds. This one can be gazed at for as long as it would take you to cross the fields, meander up the foothills, and climb those mountains. American space is different: there is more of it, and heaven is just beyond the horizonPhotograph: Ansel Adams/Courtesy Sotheby's New York