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ARTstor incorporates open access images from the Metropolitan Museum's Images for Academic Publishing (IAP) program, the DPLA (Digital Public LIbrary of America) and other open collections into ARTstor content.
This page "lists well-known sources for images for academic and scholarly use with a focus on images suitable for publication. There is an increasing trend on the part of museums to open their archives of public domain images and offer these images free for academic publications, provided they fit certain criteria such as a limited print run. Many institutions discount fees for academic publications, and fee-based image banks frequently have similar policies. Many image banks have reciprocal arrangements with non-US collections."