For a long time, pictures, glass and backing were held into picture frames with little metal triangular shaped "spikes". Now we have better stuff to work with.
If you want to put in the backer diamonds, use ones that have raised lips to get hold of, and use a special rigid Push Mate Tool as shown in the first photo.
For seating small brads, use the special pliers that gently squeeze the brads into place, called a Picture-Framing Brad Setter.
The PushMate Tool and the Picture-Framing Brad Setter came from Lee Valley Tools.