I don't think you could sue. We have moderators for a reason.
I've been to a TON of public forums, I've mad ten's of thousands of posts, but this is the first forum I've been to that didn't allow pictures. In fact, I was at a forum the night it got attacked like that, and our moderators took care of it. They never came back. I was there for at least a year and that was the only time that ever happened, and I've never seen it happen anywhere else. It's just not an issue.