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Google Art Project attempts to unlock high quality art images. The zoom feature is phenomenal. However, Google Art Project has a limited scope (for the time being), for example, there are no paintings by Francis Bacon. There is also no easy way to download images to use in research paper.
Many institutions are creating digital repositories to which they hold the copyright or items that are in the public domain, and to which the access is free. For example:
This link to a LibGuide created by Reed Library of SUNY Fredonia found public access to Artstor collections that replicate the collection in whole or in part. Where this was not possible, they suggest alternatives that are available to the public.