If you are in the US you would report it to the Cyber Tip Line at
http://www.missingkids.com/missingki..._US&PageId=169
run by The National Center for Missing & Exploited Children. If you have actually seen images or whatever you would actually be guilty of a felony if you do not report it. It helps your case if you happen to get prosecuted in the mix for viewing the stuff - if you report it immediately