You can make a super-simple picture-hanging tool from a wire clothes hanger.
Just cut a straight piece of wire about 12 inches long. Then snip one end of the wire to a sharp point and bend it to 90 degrees about a half-inch from the end.
Now bend the other, unsharpened end so it faces in the opposite direction of the sharpened end.
To use the tool, slip the sharpened end under the picture-hanging wire on the back of the frame, hold the picture up to the wall at the desired height, then press on it.
The sharpened point on the wire will mark the wall, indicating exactly where to drive in the nail.