There was a similar case in Sweden which highlighted many of the problems with current child pornography laws. It was a manga translator who was accused but was finally declared not guilty in the final instance (högsta domstolen). The picture in question depicted a topless (relatively realistic looking) manga girl standing alone on a field.
So what is child pornography exactly?
1) It depicts a child. A child is someone, real or fictional, under 18. This includes an adult pretending to be a child (also called age play). And also an adult looking like a child (willingly or not), for example by dressing in childish clothes. One tool to decide if someone looks like a child is the Tanner scale (which was used in court).
2) It is pornographic. This is of course very subjective and defined as what is commonly perceived as pornographic. An obvious problem with this definition is that something needs to contains adults (or at least teens) to be commonly perceived as pornographic to begin with. So one has to imagine to be a paedophile in order to make the decision. Which is only unnecessary sexualisation of children (for example pictures children playing on a beach becomes commonly perceived as pornographic).
The laws tend to get more and more inclusive to include more and more as child pornography. And no one wants to pull the breaks since it will get them accused of liking child pornography and being pedophiles themselves (an open goal for political opponents). While in reality the real child pornography (with real children being real victims) simply gets dwarfed by the vast amount on cartoons and teens taking pictures of themselves. Which makes it difficult for the police to legally focus their resources.
These laws are expansions of laws against indecent behavior. You are not allowed to have sex in public -> you not allowed to publicly display pictures of people having sex, or other pornographic images -> some pornographic images you are not allowed to distribute -> some pornographic images you are not allowed to possess.
It would make much more sense to instead expand the laws of sexual assault, to forbid images of those. There is not much point in determining of someone may find them pornographic or not (from a legal perspective).
One key question here is of course what the relationship is between child pornography and pedophiles committing sexual assaults. One possibility is that the pornography inspires pedophiles to commit more sexual assaults. Another is that the pornography keeps the pedophiles occupied so they commit less sexual assaults. The studies made on serial offenders point to the conclusion that pornography lessens the risk of repeated offenses. But it's uncertain if this is also true for the first offense (which isn't as easy to study for obvious reasons).