I imagine something like 1500 painting, where the king's oil painting had a few objects around him. If he had build a library, he would get a book or a bookshelf. If he made a new port, he would get a ship model. IIRC it was "photoshoped" by the painter.

Random sample from random google search https://www.alamy.com/1550-ca-spain-the-king-of-spain-charle...