Digital museum collections are a great place to find high-quality reproductions of works of art, as well as all the information you need to properly caption your images. Below you will find a selection of museums (and a couple of libraries too), which have strong digital collections online.
These other research guides by Barnard Librarians have more information on research in the arts and archives
These online reference works contain both information about artists and artworks, as well as further resources
Columbia's Avery Art & Architecture Library has over 625,000 volumes of art and art-related books. None of Avery's books are allowed to be checked out, which means they'll generally be easily available to browse. Search for Avery's books in CLIO, or browse their stacks
CLIO (Columbia Libraries Information Online) is the online catalog of the Columbia University Libraries, including Barnard Library, but excluding Teachers College and the Law Library.