Let me preface everything below by saying, 'I Am Not a Lawyer'.
I am an enthusiastic supporter of Intellectual Property rights since this is how I earn my bread, so I'm not going to suggest anybody go violating copyright.
I feel about cybertheft the way my chicken-keeping friend feels about foxes,
but without the gun.
Having finished all that prefacing, I'll do some backstory.
(If this were a work of fiction I would have lost all the readers long since.)
What is Public Domain?
Artwork and writing and Intellectual Property of every kind falls into two legal categories --
1. -- work that is protected by copyright.
That is -- somebody has rights on how the work gets used.
2. -- work in the Public Domain.
Nobody has rights to say how it gets used.
And the artist is most likely dead.
In a rather large nutshell,
in the United States,
if the picture or writing was published before 1923 it's in the Public Domain.
You can find information about Public Domain at the authoritative and well-written Stanford and Cornell sites. You want details about in-or-out of copyright? See here.
I give a site for international law way down at the bottom of the page, but those are deep waters in which I do not swim or even float on a lilo.
Use of COPYRIGHTED Images
Copyright images can be put on your blog
-- You buy rights to use the image. There are sites that sell all kinds of stock images. Ummm ... here. here. here. Sometimes this doesn't even cost much. Or,
-- You get permission from the holder of the copyright. When I write for permission, I've found blogs and commercial sites come back about half the time saying I can use their images. They want me to attribute and put up a link, which I am glad to do. Or,
-- The works are licensed under Creative Commons -- see the wiki here (thank you Larry Lessig, Hal Abelson, and Eric Eldred.) -- which means you can use them under specific conditions. Use requires attribution for many of these. You add it in small print in a caption or at the bottom of the blog.
You can copy creative commons icons here. Or,
-- Your use is 'Fair Use' alllowed under provisions of the Copyright Law. When it comes to artwork, 'Fair Use' is pretty stringently defined by the court. See Section Two here.
From a bloggers' point of view, Fair Use of images is likely to be (a) transforming the image to a new artwork, (b) using low resolution thumbnail views to talk about the art for non-commercial scholarly or review purposes. Court cases have held that thumbnails are so small and graphically inferior that they do not compete with use of images by the copyright holder. Or,
-- The work has been placed deliberately in the Public Domain.
Copyright holders can place images on sites that allow unrestricted download and use. Or,
-- It is a work produced by an agency of the U.S. Government. These are copyright to the U.S. Government and placed in the public domain. (However, works produced by Federal contractors or under Federal grants are not necessarily in the public domain.)
Use of Public Domain images
You can use exact reproductions of images of Public Domain artworks any way you want, including commercial and derivative use.
Computer files of Public Domain images can be used?
Even those from private individuals?
Even those from museums?
A simple, straightforward copy of a Public Domain painting or print can be used any old way you want.
Taking a picture of the Mona Lisa doesn't give you any rights to the Intellectual Property that is the Mona Lisa.
The ownership of the copy of a visual work, whether it is a painting, print, photograph, video recording, or .jpg file is distinct from the ownership of the Intellectual Property Rights.
Owning the physical work does not entitle one to the intangible intellectual property.
Photographing a work and creating a computer file of the two-dimensional visual image does not necessarily produce a copyrightable product. Making 'slavish copies' of works of art may require both skill and effort, but there is no spark of originality which would make the reproduction copyrightable. Indeed, the point of the exercise is to reproduce the works with great fidelity.
One way to think about the rights you acquire by making a copy is to compare it to Intellectual Property Rights to written works. If I quote War and Peace on my blog and move a few commas around, I do not then have the right to tell other people how they can use that novel. Even if I prove they copied it from my blog, I have not somehow generated rights to War and Peace just because -- hey, look, I'm so clever -- I copied it.
But the Museum site says I can't use the images.
In the comment trail of the post on Art Resources, the Excellent Annie writes, in part:
. . . If the painting or whatever is owned by a museum or is in the hands of a private collector, permission for its use may be and probably is required.
. . .
the reality is that institutions and private collectors have leverage here if only because individuals and publishers are not willing to cough up the legal fees required to challenge their assertion of . . . [reproduction rights].********
Which is a good and careful approach for a publisher of print works to take -- and something to think about if you're planning to add illustrations to your manuscript.
Museums, (may their shadow never grow less,) are as fallible as the rest of us. They give in to temptation and attempt to scoop up rights to which they have no shadow of a moral or legal entitlement.
For a summary of why they have no legal entitlement, see Public Domain Art in an Age of Easier Mechanical Reproducibility here.
I see no reason for bloggers to encourage a wholesale rights grab by private and institutional holders of the world's art heritage.
Will you get in trouble if you download a copy of Innocence Preferring Love to Wealth by Pierre Proudhon, currently in the Hermitage Museum?
I don't think a blogger has to worry about this.
It would be a poor and shabby world if bloggers were intimidated into giving up their rights to use Public Domain images.
What about the museum images of pots and swords and drinking cups?
Taking a photo of a drinking cup is a creative act. The picture of the drinking cup is copyright, even though the cup may be from 1599.
Still Life with Drinking Cup painted in 1599 -- Public Domain.
1965 photo of 1599 drinking cup in the museum collection -- copyrighted.
Copyrighted images in a museum collection should only be used with permissions of the museum. The museum's restriction on the use of their copyrighted images is found in the small print someplace on the site.
What About Works physically in Other Countries? Are they Public Domain?
Is an artwork painted by an Italian living in Russia in 1896, sold in Germany, now displayed in Abu Dhabi, owned by a Brazilian conglomerate, posted on a website hosted in Switzerland whose owner is physically located in France, and downloaded in Hawaii
in the Public Domain?
Look. Let's just not go there.
My rule of thumb is that if it's before 1900 it's probably not copyright anywhere. Information on this international law is here, but I do not delve.