As soon as they saw the terrified boy's photo three years ago, federal agents Peter Manning and Gregory Squire had the same thought: we have to save him. Assigned to the US Immigration and Customs Enforcement's Homeland Security Investigations office in Boston, their job is to track down child pornographers and victims. That single image of the distraught boy with the toy bunny became a crucial piece of evidence for Manning and Squire. It had been e-mailed to them by a Milford man who thought he was sharing it with fellow child-pornography voyeurs. His miscalculation sparked an investigation that would spread around the world, thus far leading to 42 arrests and the discovery of 140 children who were violated. The youngest was 19 days old. Robert Diduca, who sent the first photo - which he labeled cookie" in a reference to the boy - eventually pleaded guilty to the production, distribution, and possession of child pornography. In June, the 48-year-old father of three was sentenced in US District Court in Worcester to 18 years in prison. For Squire and Manning, however, Diduca's jail term did not mean case closed. They remain on the front line of a growing international effort to combat child pornography, which has exploded in volume because of faster and more potent technology that makes it easier to create and disseminate the material anonymously. Less than two weeks ago, their investigation prompted the arrest of a Florida puppeteer who allegedly discussed killing and eating children.