It's not easy to hammer brads into a picture frame to hold the backing in place. Of course, you could buy a frame maker's brad-setting tool. But they're expensive for such an infrequent task.

woodworking technique shop tip

Common slip-joint "water pump" pliers make a good substitute for a brad-setting tool. To prevent marring, wrap five or six layers of masking tape around the jaw that will contact the frame. Then slowly and steadily squeeze the brad into the frame, as shown in the drawing above.
—Van Caldwell, Cincinnati