It's always a good idea to search the network's website for help as they are more knowledgeable about their own protocol and rules. Undernet's website being www.undernet.org
With something like this however, you may wish to actually contact your local law enforcement and see if they can suggest how to take further action. Whilst of course your law enforcement cannot prosecute if they're not in your country/state, they may be able to give advice on what to do, where to go etc, or even take it upon themselves to do it. You will need substantial evidence however, and I do not suggest downloading the images yourself as that too could be unlawful.