I didn't believe for one moment that anotherjohn had made up the story, which is why I was challenging the assumption by admin that this was hypothetical.

I would not recommend taking photos of genitals, faces are far easier for the police to use for the purpose of identification. However, if you can photo somebody in the act of committing a crime or ASB in a public location then it should not be you who fears prosecution. Either way, such incidents when not hypothetical should be reported to the police.