For it to work it has to be unpleasant. In fact, the more unpleasant it is, the more it works. It has to be more unpleasant than making the decision to remove the billboards altogether.

Showing a nice animal or a 'you're welcome' message achieves the goal of removing the ad, but falls short of fixing the root of the problem: the billboard existing. As long as it exists, the temptation and incentive to put ads on it also exists.