The images are being made available via the museum’s Open Access for Scholarly Content (OASC) initiative.
From the Announcement:
The Metropolitan Museum of Art [recently announced] that more than 400,000 high-resolution digital images of public domain works in the Museum’s world-renowned collection may be downloaded directly from the Museum’s website for non-commercial use—including in scholarly publications in any media—without permission from the Museum and without a fee. The number of available images will increase as new digital files are added on a regular basis.
The Metropolitan Museum’s initiative—called Open Access for Scholarly Content (OASC)—provides access to images of art in its collection that the Museum believes to be in the public domain and free of other known restrictions; these images are now available for scholarly use in any media.
Works that are covered by the new policy are identified on the Museum’s website with the acronym OASC. Certain works are not available through the initiative for one or more of the following reasons: the work is still under copyright, or the copyright status is unclear; privacy or publicity issues; the work is owned by a person or an institution other than the Metropolitan Museum; restrictions by the artist, donor, or lender; or lack of a digital image of suitable quality.
OASC was developed as a resource for students, educators, researchers, curators, academic publishers, non-commercial documentary filmmakers, and others involved in scholarly or cultural work. Prior to the establishment of OASC, the Metropolitan Museum provided images upon request, for a fee, and authorization was subject to terms and conditions.
Direct to Digital Collection
This initiative builds upon the groundbreaking collaborative program Images for Academic Publishing (IAP) via Artstor, spearheaded by the Museum in 2007. OASC greatly expands the amount of the Museum’s images freely available for scholarly use, and facilitates direct access via the Museum’s website.