The National Gallery of Art has released its NGA Images with over 20,000 open access images. They are large resolution files that are free for download and use. The do have their open access policy posted for limitations to the use of the images. For example, they do not give users the right to reproduce the images.
They also state "As a courtesy to the Gallery and to enable others to identify and
locate information about its collections, the Gallery encourages users
to include the following credit with any use of one of its open access
images: Courtesy National Gallery of Art, Washington." This is quoted from the National Gallery of Art Open Access Policy for Images of Works of Art Presumed in the Public Domain page.
Another great social networking art & image resource that was just recently launched is Artfinder. With your own account you can create your own collections, follow specific museum or art collections, follow other people's collections, and follow art events near you.
From their about page, "Artfinder gives you access to hundreds of thousands of paintings from artists,
galleries, museums and collections around the world. With our growing number
of images, artist biographies, guides and articles, learning about your favourite
artists and works, and discovering new ones, has never been easier."
Have a great weekend!