Somewhere in the tutorials forum there's a post by… RobA, I think, on making distressed parchment with Gimp. It's easily replicated in Photoshop. A big part of it is using a grunge brush to paint on a mask over your images to make them look like parts of the vellum have flaked away. Also, I'd suggest you give your ink a red-brown tint—not too much, just enough to make it ink rather than very finely-pointed graphite. Speaking of which, those images, while beautiful, may be a bit too finely detailed to be plausible as ink on parchment.