A bit of anecdotal experience here. I pulled the head on my BMW M20b25 engine (92 325i cabriolet) b/c it was leaking oil (common problem with an older M20) and I discovered damage to two piston tops similar to the first pic you posted op. My guess was that the engine was running lean and blew apart the insulators on a couple of the spark plugs. The white ceramic bits banged around in there and dimpled the tops of the pistons. I only guess this b/c right after buying the car I replaced the plugs and noted that some were damaged, and I noted which cylinders they came from.
Anyways, cleaned them up, put it back together, and it's been fine. There is about a 10 psi split between highest and lowest cylinder compression. The cylinder walls on those cylinders were damaged by the debris too, but the engine doesn't consume oil and compression is within spec, so I'm running it that way. I've since had the injectors refreshed, which should cure whatever lean issue the engine might have been having.
1984 Stars and Stripes Powerslot
White with blue stripes. purchased 9/2012