Many museums are making selections of their archive of images of art available without copyright to the public, under "Open Access" or "Open Content" policies. These images are relinquished by the museum into the public domain, and are available free of charge for any use, commercial or non-commercial.
Many museums own the copyright for the representations of the artworks in their collection. Often these may include images of three dimentional works, which are by nature interpretive and copyrightable. Luckily for scholars around the world, many museums are embracing "Open Access" or "Open Content" policies — providing either selections, or their entire archive of images of art available without copyright to the public. These images are relinquished by the museum into the public domain, and are available free of charge for any use, commercial or non-commercial.
In circumstances where standard public domain (death of the artist over 70 years ago) is not available, most often in the represntation of three dimentional works, we look to these institutional collections for images of the artwork. On Trivium, these artworks are labled "Institutional Open Content" under the liscense field. For more information about that artwork's specific liscence, follow the citation link to that artwork's Museum website.
More and more institutions are joining the Open Content movement. Below we've listed some of the amazing organizations of open content libraries we've used:
The Getty — Open Content Program
National Gallery of Art — Open Access Policy
Metropolitan Museum of Art — Open Access for Scholarly Content
Freer Sackler Galleries & Freer Study Collection — Open F|S
The British Library — Welcome to the Commons!
For more amazing open content as well as news and updates from the open access movement, check out openculture.com.