|Note this page is still under construction.|
When someone comes into a country (the United States has by far the most extensive program) with a computer that has pictures on it, or mailed discs are taken for examination, or an ISP reports that someone is sending child porn as attachments to an email message, no person has done this by looking at every picture attached to every email sent through Google's or whoever's servers, to see if it is illegal. Only a machine can do this fast enough.
But a machine has no way of knowing what the picture contains. What it does instead is compute a checksum code, a string of nonsense that according to the rules followed, could only have come from one particular picture. For example, a picture's checksum (written in Hex, base16 numbers, look it up in Wikipedia) might be:
E%¥syb358@s4457evr (made-up example)
Law enforcement, like Homeland Security, has a list of the checksums of millions of illegal pictures. These come from seized computers. The list is distributed to ISPs, email processors and the like, and when your laptop is taken for inspection at a border, software will tirelessly make a list of the checksums of each picture on it. This list is then compared with Homeland Security's list of checksums of illegal pictures to see if there are any "hits". If so, an alerted human will eyeball some of them. Out come the handcuffs. You're under arrest.
This is why there is a never-ending string of child pornography cases. That's a type of crime that can be detected using automated means, so it doesn't take so much of law enforcement's resources. Catching terrorists making plans, or online financial crime, is much harder. But the child pornography cases are the low-hanging fruit.
Don't have child pornography (pictures). You'll probably get caught.