I am putting together a metal picture frame, and I was hoping to find a hardware piece which would serve as 2 purposes: 1) to press the foam core backing against the frame, and 2) to use to hang the picture. Several pre-made frames come with this piece. It's like a little U-shaped metal prong- you just pinch it together to fit under the back of the frame moulding, and it has a little sawtooth hole which can be used to hang the picture. What is this metal piece called? Is this the best way to hang my picture?