it's not exactly objectively measurable.
Yes and no.
A picture can say a thousand words, but if the first of those words consist of "lol", "kek", "HAHA I TOO AM IN AGREEMENT", "oh", etc, then it's a shitpost clear as day.
Yes, discussion can spring up anywhere, but there should be some
effort thrown into an OP and I think that distinction is what defines a lock worthy shitpost and one that is allowed.