In a way I think you made my point for me.

Leaders change the art because they are looking for a new effect, or some effect. In your example of Henry VIII posing with a portrait of Henry V behind him. How many times do you think that would work before his councillors would sigh and wonder why the leader thinks that still works. They may still agree with the policies proposed because they are good policies, or from fear, but the art would no longer have an effect.

As you mentioned, the art is changed from time to time depending on the situation. So it follows that even you feel that the art loses it's effect after a time.

Do you reallly believe that perisians pass by the "arc de triumphe" and get bleery eyed on a day to day basis? No, we are adaptablecreatures and after a while it's just another peice of landscape. Something you may take pride in when talking with foreigners that aren't being overwhelmed enough for your tastes, but nothing you really even take notice of in your day to day life.

Maybe my Will save is just too high, but I just don't see how putting a statue in town square could have a permanent effect on morale of an entire province or town.