Laverite, I very much understand your wanting to avoid copyright infringement. I suppose that the WT lawyers would get every penny they could out of you, and that is not what anyone wants. Except, possibly, them.
But, let me dream on, will you? This could be a battle worth losing. Their suing for copyright infringement could backfire. All they could claim would be that you didn't have the right to put that image on the billboard, and would want it removed. But, their very taking action would mean that indeed, they printed that Awake!, and, indeed, they did say that no one should be forced to choose between religion and family. It would lend legal credibility to the claim that shunning is not something people do on their own. And then they could not prevent you from REMOVING the image from the billboard and putting something like "removed for copyright infringement" instead. They could not prevent you from finding a friendly journalist to report on the issue. I don't think this is what they would like in terms of Public Relations.