This is very simple. You can either get a frame hanging kit
at your local hardware store, hammer the nail into the
drywall where you want to hang the picture, and you're done.
It doesn't sound like there's anything on this metal frame
where you run a length of picture frame wire that would rest
on the hook & nail in the wall. If that's the case, just run
your finger behind the frame top horizontal member, and
approximate the center. Then gently hook the top horizontal
part of the frame on the hook or nail in the wall. Center the
picture slightly so it appears to be straight. Simple.
Make sure the nail is hammered far enough into the wall
so it will support the weight of the picture. 2 to 4 lbs.
is fairly light.