Not an expert but agree those are plaster walls. We are living in our first plaster wall house. It was built in 1945. Ours is a rental so I am not doing anything major, but from what I have read, the best thing for plaster walls is picture railing.

You can google a picture but it is a trim that goes around the room at the same height as the tops of the door and window casings. The top of the trim is away from the wall by half an inch or so. You then use hooks with weighted wire to hang your pictures in various locations and heights. I would personally love this and the ease of changing pictures around when I felt like it.

But it isn't for everyone, so if you want to go more contemporary then use screws not nails and a battery or electric powered tool to screw them in. I did this in my mom's home to hang her drapery rods and learned the shorter the screw, the easier to get enough force to get them going. Basically press the screw in as it turns until you start seeing the plaster turn to dust and then you will know it is about to get through. Patience is required and lots of arm muscle fatigue if doing a bunch and not used to it, so just plan to hang a few things at a time rather than thinking you will get all the pictures in the house up in an hour.

Looks like plaster. Purchase a capable stud finder for determining the best places to mount curtain rods. There's a range of products for mounting hooks for pictures in plaster.