"Cheese", or "say cheese" is what UK and US people usually say when someone is taking a picture, because it makes your mouth to show a 'smile-like' shape.
In the late 19th century, it was more nice to keep the mouth small, which led to photographers to use the phrase "say prunes" (so the 'duck face' is old like that!)
I've also heard another instructions like:
"Smile" / "Smile for the camera", "Look at the birdie" or "Act serious!"
But the interesting thing is to find out the common words said in