A number of stamps were printed in the early 1940s with drawings or paintings of images in a bold, Mexican Art Deco style, in a large square format with a common appearance. Many of these were the artwork of Francisco Eppens Helguera, a Mexican artist whose images were used on numerous Mexican stamps in the late 1930s through the early 1950s. Eppens also created the small, but iconic, 1939 postal tax stamp depicting a man attacked by a giant mosquito, issued to raise funds to combat malaria. Beginning in the 1940s, Mexico issued a great variety of stamps in different styles and sizes (often large), typically depicting persons, places, objects or events connected with the country and its history. The stamps were mostly printed by photogravure, but lithography reappeared and became common for a number of issues beginning in 1992. Mexico's stamps were printed in one or two colors until the stamps commemorating the 1968 Summer Olympics held in Mexico City in 1968, which were the first multicolored stamps issued by Mexico. These stamp issues included several series of stamps with common design elements which were added to over a period of time, especially as inflation increased postal rates. Several of these series have been popular with philatelists, foremost of which is the Exporta issue, discussed below. Annual stamp production increased significantly during the late 20th century.