http://www.govart.com/hardware_hardwall_C.html
You can get them at Walmart, Home Depot, etc.
Just tap them in gently. Use a flat head screwdriver to pop them put when you don"t need them any more.
Wire a washer to the hook on the back of the frame to hang smaller pictures. For larger pictures, use hanging wire.
You can also use them to hang drapes.