They were limited to use models drawn from the dead.
Well, there was a lot of building going on, and many times the large stones would fall on or roll over the citizens lollygagging around, and when the stones were removed off the dead folk, what they saw was flat people, with one arm in front and one in back, in the act of running for their lives from under the rock as it fell.
Skidadelling, if you will.
These were the models from which they drew inspiration to depict on the walls.
Sometimes there were dogs, and birds, snakes, and such as well.
That is so brilliantly logical that it has to be true.
Not sure about that, but something similar was the origin of the panini.