ARTstor is a large repository of high quality images. The other resources are institutions, libraries etc. who provide free databases with searchable images.
A digital library of over 500,000 images of art, architecture and archeology from a wide range of cultures and time periods to support teaching and learning in the humanities and social sciences. Artstor's high-quality collections and key functionality are now available via JSTOR.
Access to unique and historically important digital collections: over two million photographs, documents, letters, artwork, diaries, oral histories, films, advertisements, musical recordings, and more.
Find paintings, drawings, and photographs, in addition to manuscripts and decorative arts from the J. Paul Getty Trust. Includes over 4,600 digital images.
Provides access to descriptions and inventories of thousands of primary resource collections maintained by archival units across the Smithsonian. The collections document the history of art, culture, music, flight, space exploration, science and technology, and native cultures in the United States, as well as history of the Smithsonian itself.
Art reference works including Grove Dictionary of Art and the Benezit Dictionary of Artists. Over 200,000 entries span ancient to contemporary art and architecture. Includes numerous images.
Museums
Museum often have scanned images of art that they hold in their collections.
The Louvre' collection is grouped into eight departments: Paintings, Egyptian Antiquities, Greek Etruscan, and Roman Antiquities, Near Eastern Antiquities, Sculptures, Decorative Arts, Islamic Art, and Print & Drawings.
With over two million works of art, The Metropolitan Museum of Art in New York has an "encyclopedia collection"; 400,000 of these works are viewable online
The Museum of Modern Art in New York features modern and contemporary art with approximately 200,000 works from the last 150 years; 60,000 artworks are viewable online.