Wow. That's a crazy high res source of the Mona Lisa. Nice find.
I could give this a shot, but I'll be honest -- for reasons that are kind of hard to explain, I'm trying to kick the Photoshop(R)(TM) habit. Not photo editing in general, just Photoshop in particular. Gimp lacks adjustment
layers, which is like having my arm cut off. They've been saying they'll add them for, what, at least a decade now?
Seriously **** Gimp
I may experiment with Krita, which I hear
does have adjustment layers, but I haven't really used it before, so I don't know if it's actually any good.
So if I give this a try, I'm not really sure what tools I'm going to have to work with, or how well it will turn out. More to the point, I don't know how fast I'm going to be able to work, so deadlines might be an issue if this is something you need in 2 or 3 days. How soon do you want this done?