If you want to hang the pictures in the style that would have been in use at the time the house was new, you can get a molding shape at the home centers that allows you to do this. I believe that the name is "picture rail". You normally install it a foot or two down from the ceiling and use coped joints at the inside corners, just like you would for base mold or crown molding. Since it is relatively small, it is pretty easy to install and you should paint or stain and clear finish it before installation. Be sure to snap a chalk line from every corner of the room to the next corner so you can keep it at the same level around the room.
Once you have the molding up, there are hooks that go over the molding and you run a wire or small chain down from the hook to hold the picture. The molding and wire makes it easy to reposition a picture or add more. One benefit it that if you keep moving them around, you don't end up with sun bleached or dirty walls marking where a picture has been hanging for a few years.
Of course, you can just get the flanged nails for driving into a wall at the home center that are made for holding pictures also.