I would say there's no "rule" per se, but I will also say that the assumption I made to this point is that, other than those in the unicorns thread, I figured the pics being posted were for posters that are owned. Otherwise, we only have to go to impawards.com to see posters and throw them into this or that category.
I guess putting it another way, what's the point of posting pictures of posters that you don't own? It's like window shopping. Dry humping. Low Cal Sweetener. Low fat cookies. There's no payoff.
I'm not going to make a rule, and I don't presume to even have the right or power to make a rule like this. But, I know I would consider the fact that if I started posting posters of things I don't own, I would expect that someone in the group is going to question just what the hell I'm trying to accomplish.
I'm sure this can be argued the other way. At the least, postings by others of posters that caught their eye but that they do not own, may broaden my own collecting horizons to things I have not seen or considered before.
Idle thoughts and ruminations.