According to the US Customs Service: "There is no easy formula for discovering and identifying a consumer or purveyor of child pornography. However, if you have information about or suspect this type of illegal activity, contact the U.S. Customs Service as soon as possible. Call 1-800-BE-ALERT.
The U.S. Customs Service also works closely with the National Center for Missing and Exploited Children to combat the proliferation of this disturbing material. You can also report suspicious activity relating to child pornography to their "Tipline" at 1-800-843-5678. In addition, the Interpol website gives an email address of "email@example.com" as an address to report suspected child pornography. The address accepts mail but doesn't provide any feedback on what action will be taken. Given this uncertainty, reporting it via email isn't a bad thing to do, but we would suggest people in the US contact the US Customs service first.
Canaidan Residents should review Cybertip.ca for information and help.