The magazine began to feature some pages of color photography in the early 1930s, when this technology was still in its early development. During the mid-1930s, Luis Marden (1913–2003), a writer and photographer for National Geographic, convinced the magazine to allow its photographers to use the so-called "miniature" 35 mm Leica cameras loaded with Kodachrome film over bulkier cameras with heavy glass plates that required the use of tripods. In 1959, the magazine started publishing small photographs on its covers, later becoming larger photographs. National Geographic photography quickly shifted to digital photography for both its printed magazine and its website. In subsequent years, the cover, while keeping its yellow border, shed its oak leaf trim and bare table of contents, to allow for a full page photograph taken for one of the month's articles. Issues of National Geographic are often kept by subscribers for years and re-sold at thrift stores as collectibles. The standard for photography has remained high over the subsequent decades and the magazine is still illustrated with some of the highest-quality photojournalism in the world. In 2006, National Geographic began an international photography competition with over eighteen countries participating.