Use a 4D or 6D nail.

 As long as your painting is less than about 5 pounds, a regular finish nail will be sufficient to hold it, even in drywall. The nail should be around 1 ½”(3.8 cm). Definitely don’t use a nail that is over 2” (5 cm) because it will end up being too long.[2]

• Don’t use a nail that is less than 1” (2.5 cm) because most walls will be at least that thick and the nail won’t be long enough

• If you know that your walls are made of brick or plaster, use a screw rather than a nail. Use screws that still have a small head on them.