Learn the correct terms for US copyright law. Life of author + 70 years only applies post 1978.
Life plus 70 years only applies to unpublished work under specific circumstances and to newly published work after 1978. You have ignored entirely any consideration of the Copyright Law of 1909, which covers most of the paintings you claim are "public domain." This destroys your site's credibility.
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Law of 1909 covers works published with a © notice in the U.S.