Terminology
Since it keeps coming up, I'm going to harp on the terminology issue again.
Child pornography, indecent images of children, child abuse images. People tend to treat these terms as interchangeable, but they are not.
CHILD PORNOGRAPHY (or variations thereof: child porn, kiddy porn) is the most common term and probably closest to what people mean most of the time. Obviously this term refers to material which shows a child and is pornographic (sexually explicit for the purpose of arousing the viewer). Some people (including the IWF) object to this term on the grounds that by analogy with adult pornography it implies the child is a willing participant. (Which in reality may or may not be the case.)
INDECENT IMAGES OF CHILDREN is the legal term in the UK. "Indecent " is even more subjective than "pornography", but I think we can agree that the former includes all of the latter plus some content that is not explicit enough to be considered pornography. This is one of the strictest laws around on the subject.
CHILD ABUSE IMAGES is the term preferred child protection activists and organizations such as IWF. Logically this would mean an image of a child being abused, which understandably provokes an emotional response. Unfortunately people rarely mean that when they say it. Nobody takes this to mean images of non-sexual abuse (except in Australia). Instead they mean "child pornography" or even the broader "indecent images of children". This is inaccurate because there is nothing in the definition of those terms which indicated there must be abuse.
Mixing up these terms causes quite a bit of confusion. For instance, in this case IWF says "we block child abuse images" and then goes on to block a picture because they deem it indecent (and thus illegal) and people seeing this conclude that "IWF accuses Wikipedia of hosting child pornography". Wot?
The Virgin Killer cover may be indecent, but it is not pornographic or an image of child abuse. If a child decides of his or her on free will to masturbate in front of a webcam, the result is child pornography, and indecent, but still not an image of child abuse. If the police are caught on camera tazing a 10-year-old, that's an image of child abuse, but not indecent or pornographic.
So, GET IT RIGHT people!
Here's a helpful Venn Diagram: (IIoC {CP [CAI } ) ]