I can't even imagine why you would think this is a good idea. Sometimes the best advice a lawyer can provide is to calm down and act like an adult, even if your wife isn't.
If you need a legal reason not to move forward, they are numerous. Including intentional infliction of emotional distress, copyright infringement, harassment, etc.
Take the money you'd be spending on billboards and use it toward a downpayment on a sports car like a normal divorcee.
Your proposed course of action is a bad idea for a lot of different reasons. If you don't have a divorce lawyer yet, find one that's a shark and will zealously represent your interests in the divorce. Also make sure they can advise you on things like this in more detail, explaining why it would be a very bad idea for you to do something like that billboard.
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There is also copyright infringement since you didn't take the photos.
Avoid the added stress .... don't purchase the billboard.
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