Just to address the original points of this topic.

[quote=@Ruby]

Should we have a policy for this?

[/quote]

With just three instances of it happening over the course of two years, I don't think it's a very big problem. You should probably have a set of protocols you follow. But it sounds like the mod team already has one in place. If they don't want their image somewhere, and they can prove the image is theirs, simply remove it. It doesn't need to be any more difficult than that.

[quote=@Ruby]
What if it's a new post?

Obviously we'd contact the member with the image in their CS and ask for it's removal, or should we? 
[/quote]

I think you should. You almost have to. If you're worried about a lawsuit, something needs to be done. At least if they actually do have copyrights. From what I hear, they are quite expensive to obtain. But as others have said, it's the polite thing to do.

[quote=@Ruby]
What if they say no? 
[/quote]

Perhaps it would be best if it was said in the rules somewhere. But really, the forum could be put in danger, since it is essentially what is hosting the image. They simply must comply. There's not much room for debate.

[quote=@Ruby]
Should we edit their posts ourselves?
[/quote]

If not outright delete it. Though I would give the user a chance to edit the post themselves.