What is the biggest picture or drawing in the world?

It's not known for definite, but these are thought to be the Nazca Lines of Peru.

These are a series of pictures, shapes and geometrical designs that are carved into the desert soil of the Nazca Plateau, an arid desert region stretching some 50 miles between the towns of Palpa and Nazca in the Pampas de Jumana region of Peru. There are over 200 images, ranging from simple rows of parallel lines through to complex geometrical shapes and elaborate pictures of animals, including hummingbirds, monkeys, fish, spiders, llamas, lizards, snakes and even human figures. The largest of the pictures measure over 660 feet across!

They were thought to have been created by Peru's ancient Nazca People between about 200 B.C. and 700 A.D., using the simple technique of removing the reddish, stony upper layer of desert soil to expose the whiter, chalkier soil beneath. Their purposes probably varied- the straight lines may have been to indicate the flow of water courses, or be connected to religious rites to summon water to such a dry, parched part of the country. Some of the animal pictures may have been religious or fertility symbols, whilst the more complex of the geometrical shapes could have been a form of astrological calendar or have been used to mark time. The dry climate of the region ensures that the Nazca Lines remain in good condition even today, and they are now declared a UNESCO World Heritage Site by the United Nations.