Old plaster is typically very brittle. When you
hammer against the wall to drive nails and hangers
you run the risk of creating a crack, once you have
a crack they tend to grow and reappear after patching
through the changing seasons. Also the strength of
the plaster is partially the mushrooming(best term
i could think of) of the plaster through to the
back side of the lathe. Hammering on the wall tends
to bust these tabs away and weakens the plaster further.
It was common practice in those days to install a
piece of wood moulding all the way around the room
about 1-2 feet below the ceiling. This is known as
picture rail. Pictures and other items are then
hung with decorative cord and hooks. You can move
raise or lower any time you wish without tearing
up your walls. Many vintage homes had picture rails
but they've been removed over the years.
I used a nail gun to install mine and it prevents
damage when installing. You can find the cord and
hooks at craft/hobby stores.
It also looks excellent and is true to the period
of your home.