Read the literature on rigor yourself. Plenty of info out there. Summary: Rigor can be seen in a little as 10 minutes after death, although it generally starts later, and it is generally complete about four to six hours later. The onset and rapidity of rigor varies with temperature, age, sex, physical condition, and muscular build.
In particular, rigor sets in more quickly in a warm environment (the Middle East in summer, for example) than in a cold environment.
Rigor mortis starts on the face and gradually progresses downward. The child in that photo does not show signs of full rigor throughout her body. Her right arm, hips, legs, feet, and neck all still appear to be flexible.
Just in case you want a reference, here's one to the Columbia Encyclopedia.