Otenth, taken from Wikipedia here. I quote: Under United States copyright law, originality of expression is necessary for copyright protection, and a mere photograph of an out-of-copyright work may not be protected under U.S.copyright law. This photograph was taken in the U.S. or in another country where a similar rule applies (for a list of allowable countries, see Commons:When to use the PD-Art tag#Country-specific rules). This photographic reproduction is therefore also in the public domain.

Assuming this is licensed as a 'PD-Art' imagine on wikipedia, we may assume that:

1. The photograph of the painting was taken more than 25 years ago in spain. It's that photograph or a scan of it in a book. (The Ley de Propiedad Intelectual (LPI 1996) [7] (1987) provides 25 years of copyright to "mere photographies" or similar reproductions, beginning on the 1st January of the following year of the creation.)
2. The photograph was taken on USA soil. (Less likely. As far as I know, the Garden of Earthly Delights never moved to the USA for exhibition.)