This can't be enforced automatically, and it would be difficult to enforce through moderation.
I believe a more false statement cannot be made. I guarantee as soon as people know the image size rules, you will never see more people ratting on other Bitcoin Forum users than ever. If no one is ratting, they truly do not mind the image size. I assure you though, someone will always notice. Been on forums before that had self regulation of this specific kind that worked.