When taking a photo, South Koreans say “kimchi” instead of “cheese.” In English, photographers usually say “Say cheese” to make people to smile in pictures. In Korea, when photographers count 1,2,3 or 3 to 1, photographees say “kimchi” by themselves. As the pronunciation of “cheese” makes person’s mouth to smile, which of “Kimchi” also makes person to smile.

Source: http://www.kpopnews.org/2012/09/why-do-koreans-say-kimchi-when-they-are.html