Hello!
My name is Bill Baird. I have had a more than passing interest in Pictish and Celtic stone markings for almost all of my life (a longish one so far) having been an ardent long distance hill walker when younger.
I had the beginnings of an idea many years ago which has not developed into a full blown theses, as yet, however things change.
I have suspected the carved stones represent way guides for people and places scattered around the area, the population being scattered and sparse in those days.
They would give indicators as to the tribes and facilities available and in specific directions, so if you wished to know information relating to say the "Snake" tribe for whatever reason the marker would point the direction in which the main settlement lay, it may have also given an indication of distance.
Looking at some of the Scottish ones, one which is rather obvious I think, which is called an Elephant or Pictish beast, is in fact rather obviously a dolphin.
They represented with great exactness other animals and objects - eagles and salmon for example - therefore why should they be so far out with the "Elephant?" The spout can be observed coming from the top of the head (Blow hole) head and the curved "Feet" the implied fins, which are not often seen excepting when the animal jumps, the lengthening of the snout reflecting the dolphin can also be observed.
Semi circular markings would represent a settlement with an enclosure round it and the rods and arrows giving the direct marking for the settlement, the quality and amount of detail setting out the size of the settlement, this would represent a direct investment in time and effort by that collection of individuals which would be reflected in the quality of the end product.
Perhaps the foregoing shall give something for you to passingly think about.
Bill Baird.