I mean, that is the nature of conversation is it not? I've never had a conversation where one topic did not lead to another, that is generally how conversations evolve. Maybe create one of those threads about specific types of photos, or a trivial thread, if you do not want conversations to naturally evolve.
And isn't discussing threads being derailed, also derailing the thread? It does not pertain to the topic at hand, and is clearly now leading to the conversation evolving in a different direction.