Drawing from the concept of the windmill, kingdoms should have a machine, structure or giant tub that makes production of sketches easier. A downside could be that they would be available only to citizens and would be taken directly from their hfs. And the number of contours could also be limited to prevent reproduction of complex art.
If I had a kingdom, I'd want to see some of my vassals wearing the kingdom's colors and hanging banners on their gates. Advanced players certainly wouldn't because they have their own sketches, but underdeveloped noobs and vassals could find that interesting.