Sorry, you missed my point completely. They are potential criminals because until they are found guilty they are innocent. They are only incriminating themselves if they were actually breaking the law. I know the police and most probably the government would absolutely love to not have to bother with all the hassle of actually proving someone guilty, but thankfully, just because they or you decide someone is guilty, it does not make them so. A court decides that. I can think of a number of reasons why a photo of someone with a weapon outside would not make them guilty of breaking any laws. Maybe it was taken outside of Scotland or maybe it was taken on private property, which I'm assuming the law doesn't cover as it is not a public place.

Anyway, I'm not against the idea, if it helps then good. I suspect there may be better ways to reduce knife crime but I'm fairly sure they would rely on having more police and so more cost, certainly a lot more cost than sticking a couple of trainees in front of a computer.