Eric Felten writes a strong piece in the Wall Street Journal explaining why reproductions of public domain works have not copyright protection in the US:

U.S. law provides a precedent almost eerie in its similarity to the current controversy. In the late 1990s the Bridgeman Art Library Ltd., a British company that licenses fine art images, sued Corel Corp. for selling a CD-ROM featuring public-domain images taken in part from Bridgeman’s collection of copies. The company claimed that the effort, skill and judgment that went into making high-quality photos meant those images were covered by copyright, even though the paintings being copied were all of moldy vintage.

Although “doubtless requiring technical skill and effort,” the U.S. Court for the Southern District of New York ruled, “exact photographic copies of public domain works of art [are not] copyrightable under United States law because they are not original.”

But one of the key issues with the National Gallery is that Derrick Coetzee didn’t merely download the images, he had to unscramble them and reassemble them to construct the High Resolution images, according to the BBC News:

The gallery also explained how Derrick Coetzee was able to obtain the high resolution files from its site. They were made available to visitors using a “Zoomify” feature, which works by allowing several high resolution files to be seen all together.

It claims Mr Coetzee used special software to “de-scramble” the high-resolution tiles, allowing the whole portrait to be seen in high resolution.

This is important because, as Felten explains, museums are aware of their tenuous copyright claim but have gone to the effort of imposing contractual restrictions to limit access to the work they’ve created:

Even so, it’s not uncommon for museums to try to protect an interest in the reproductions they make. Go to the New York Public Library’s database of images, and you’ll find it more than happy to send you digital files of artwork—as long as you agree to limits on how the image can be used. The National Portrait Gallery has language on its Web site setting such conditions and limitations and may well try to enforce it as a binding contract.

Whose Art Is It, Anyway? (Wall Street Journal)