The way the boy is sitting in the picture says that he is sitting on his leg. But in some texts I've found that this way is called 'sitting on left foot'. However I feel if we say 'sit on left foot', it'll sound as if one is sitting on his left foot squatting.
'Sitting on hams' may be used too, not sure though. What is indeed the part this pictures shows the boy is sitting on?
To me 'leg' seems to be fine.