Are photographs of public domain images protected by copyright? According to one judge, the answer is: "no". See The Bridgeman Art Library, Ltd. v. Corel Corp., 36 F. Supp. 2d 191 (S.D.N.Y. 1999). The Bridgeman case is a United States District Court case. As such, it has no value as precedent anywhere (except for the parties involved in the case). It is well reasoned, however, and relies on Second Circuit and Supreme Court cases which do have value as precedent.
Defendant Corel Corporation was sued for selling a CD compilation of fine art in the public domain. The Bridgeman Art Library, Ltd., which markets color transparencies of museum-quality reproductions of the same works, claimed copyright infringement. Plaintiff claimed that some of the images on Corel's CD must have been derived from Plaintiff's transparencies because no one else had permission from musuems to take the photographs.
Plaintiff, a British company, relied on its UK copyright registration, which gave rise to a legal presumption that the work was original and copyrightable, and an 1869 Queen's Bench case indicating that photographs of public domain art can be considered "original". See Grave's Case, 4. L.R. 4 Q.B. 715 (1869). The federal court for the Southern District of New York (the district which includes New York City) court held that U.S. law applied under the Berne Convention, an international copyright treaty. The Berne Convention applies the laws of the forum state, that is, the country where the lawsuit is filed. The court also also noted, however, that a legal presumption is merely a presumption, and as such can be rebutted by evidence that the work is not original. I'm not sure this is an accurate statement of UK copyright law, but, if the plaintiff sues in the U.S., U.S. law applies under the Berne Convention.
Under U.S. law, an exact copy --a "slavish copy"-- of an image in the public domain is not "original" and therefore not subject to copyright under the Copyright clause of the United States Constitution.
This is not to say that photographs cannot be copyrighted. They most certainly can. Also, photographs of images still under copyright --virtually all 20th Century art-- are still subject to copyright. Collage would be copyrightable, as would be entire collections or arrangements of public domain images. Moreover, a "creative" photograph of a public domain image, one that involved aesthetic judgments about lighting, filters, development and so forth, might possess the requisite "originality" to get copyright protection. In particular, photographs of buildings and sculpture require the photographer to address questions of lighting, arrangement, composition and so forth in order to depict the three-dimensional art in two dimensions, and thus may qualify as "original".
The images in Bridgeman, however, were museum-quality reproductions: precise copies of the public domain originals. As such they were not original, and not copyrightable.
CAVEAT: Appropriating museum-quality images for commercial purposes is a bad idea. Bridgeman probably could have used alternative legal theories to successfully sue Corel --unfair trade practices come to mind. However, the case does establish that public domain images stay in the public domain even when a museum has charged a fee to photograph the image, which should encourage fair use and non-profit dissemination of the cultural heritage of the human race.
Conclusion:
So is it legal to post pictures on the web? Yes, if it is an exact copy of an old (pre-1900) painting, engraving or other flat, two-dimensional artwork.
Case text: www.law.cornell.edu/copyright/cases/36_FSupp2d_191.htm
Discussion of case and its chances of being reversed on appeal (none): http://www.panix.com/~squigle/rarin/corel2.html
General discussion and links to related issues:
http://www.funnystrange.com/copyright/bridgeman.htm | 
