(1) The main reason why pictures of naked children are illegal is because it involves the actual exploitation and brutalisation of a child to make them. If the picture in question is a drawing, then this doesnt apply, assuming it wasn't drawn from life.
(2) Second issue is how sexualised the picture is. There is nothing wrong with pictures of naked children per se - Western Art is full of Cherubs, Cupids and frolicking nymphs and so on to Nirvanas Nevermind. It is when the image is sexualised that it enters the greay area.
(3) The most controversial element is the sexualisation of young, but not actually pre-pubescent girls. We, have, for the sake of convenience, set a bar at 16 below which it is illegal to go. But of course this bar has no basis in nature: An individual only has mental problems if he is sexually interested in girls/boys below the age where they become sexually mature (13 or so). It is perfectly natural to have some sexual interest in Girls/boys after that age, though of course not entirely indicative of a healthy adult attitude to matters sexual. (i.e. it's a bit sad, but not actually sick)
The Japanese anime type stuff seems to be playing into this area of male sexuality. the desire for young, skinny, weak-looking teenage girls. Its not entirely healthy, but as long as the girls in question are sexually mature, and no actual girls are involved in the making of the pictures, I wouldnt worry about it too much.