Top pic looks like some sort of solid small object made dimples in the piston. Second one looks like some sort of erosion caused by detonation or coolant/water.
While there may be some irregularities on the tops of the pistons, it really is the damage and wear to the sides of the pistons and the condition of the cylinder block bores.
Does the top ring of the piston(s) rotate freely? Any other abnormal marks in the bores? Any marks in the cylinder heads (of the corresponding cylinders?). If not, those pistons are re-useable. Any other suggestions would require additional information.
You already have the heads and pan off, so how far are you going? If the engine ran fine and compression was good, what made you decide to go after the internals of a good engine? Might as well rebuild now....