Called "screw-eyes" these are the most well known type of hanging mechanism for the backs of frames with cord attached for hanging.

Inner Diameter of screw-eye: 5mm
Length of screw-eye: 19mm
Quantity: 50
Designed for smaller frame sizes up to approx A4 size

Simply make a hole in the frame, start screwing in the "eye" and then using a skewer or a  thin rigid tool poke through the hole and rotate clockwise till the screw-eye goes in enough to NOT see the thread. much easier than trying to do it with your fingers, especially if the wood is hard.

Framingonline Tip: cord is generally attached ONE THIRD of the way down from the top of the frame.