Pencil if the medium is matte. Go to a gallery or art museum and look; virtually all will be signed in pencil. It's the most archival writing instrument because unlike all forms of ink, pencil will never, ever fade. The graphite will literally outlast the paper itself. Also unlike many inks, pencil will not damage the paper because it has no chemicals in it acid or otherwise. It will smudge if rubbed, but you shouldn't touch the surface of a print anyway.