On July 10, 2009, the Wikipedia user Dcoetzee received a legal document from Farrer & Co LLP, demanding the removal of several thousand images of public domain art that had been uploaded to Wikipedia.
A copy of the legal letter received by the user has been posted to Wikimedia here:
Some More Links on This Issue
Discussion of the Issue at Wikimedia
Protest the NPG's Legal Threats
Consider contacting the NPG and asking them to back off from this and similar threats in the future.
Here is the Contact page for NPG.
NPG Contact Us Page
Consider contacting their complaint and suggestion department, listed at the bottom left of the page.
Here is a form letter you can use or edit to your liking for submission to the NPG.
Proposed Complaint Letter 1.0
I also welcome suggestions for improvement or alternate letters to post here.