For everyone who's wondering what they're talking about: the background art will change when you build one of the upcoming Ancient Wonders. You can read more about it in the Ancient Wonders
announcement (click).
As this idea is more about a new storyline involving the background art than about changing the background art, I'll leave this thread open, even after the Ancient Wonders feature is implemented. But it's great to hear that you're happy with the upcoming feature!