I'm wondering why a person would have the impression or suspicion that it would be illegal to post a photo of a legal item.
I mean, I don't even think it's illegal to post a photo of an illegal item. (Obviously, you couldn't post child pornography, but then, that wouldn't be a photo of an illegal item, that would be the actual illegal item.) But let's say you came upon a photo of a liquor still out in the woods. That would be an illegal item, but why would you think showing a photograph of it on the internet forum would be a crime?
Besides, we've seen probably thousands of photos of firearms on forums like this one and others. I bet if it were a crime we'd have known it by now.
-azurefly