I could be wrong, but I think there's far less of a market for producing photos of brutal violence than there is for producing photos of illegal pornography - the law is really about trying to kill the market, rather than it actually mattering that they are looking at the photos (really the only reason to punish people for looking at them is that by stopping these people you can help take away the incentive for people to produce them in the first place).
Edit: In response to tomp's comment about how cartoon images can be illegal... yeah, I don't understand that. Maybe there's scientific evidence that paedophiles who look at cartoon images are more likely to become sex offenders than those who avoid all pornography? Even then, not sure it's the right approach... but who knows.