I just bought a house built in 1939, how can I hang up pictures?

Before putting your screw or molly into the wall, simply apply about two, 2 inch pieces of masking tape in the form of a plus sign over the point that the nail, screw etc. will be inserted through the plaster. The tape holds the plaster together.

3M Command hooks.

In the time period your house was built homeowners installed picture rails to hang small to midsize pictures and mirrors. Then they would attach fine picture wire to the back of the picture the length needed to position it the height they wanted It to hang on the wall.

Of course that means you would still have to drill screws through the plaster into the studs to attach the rails. But once that is secured you can move your pictures around at will by attaching small screw hooks along the rails to hold the wire.

I agree with William, command hooks work wonders!