I’d suggest reading Paula Bray’s paper (http://www.archimuse.com/mw2009/papers/bray/bray.html, as mentioned by Seb above).

Were the images on Wikimedia actually attributed to the NPG? Does Wikimedia re-write the rights around the images they have reconstructed? Check Jim’s comment on http://www.sydneyobservatory.com.au/blog/?p=1975 If someone wanted to purchase a high res version, print or license, would they know who to ask?

I should really post this to the MCG list, as I think these discussions are better held centrally, but I’m rushing out the door…