You can caulk the mirror to the wall. This will keep it from moving. If you still have the old mirrors and they are the correct size you can easily frame them yourself. No special skills needed.

I would do mirrors in frames. Hang them with a hook on each side so they won't move around. If you caulk them to the wall & you every want to remove them, you could tear off the wall board. You can find 3 different mirrors so every room won't be a carbon copy. You can also cut the old mirror to a size you want & use molding to make frames. You would use metalic paint or paint them a solid color.

Good idea lucidos. This would do the trick without being a permanent solution.

Actually Lucy, you just use a razor to break the seal when you want to take them down. Caulking them up resolves moisture getting behind the mirror and insures someone does not elbow it off inadvertently.

lucyhomedecor, I think removing caulking from a wall may cause minor damages but it is easy to fix with minimal plaster and sanding before you re-paint.