Sources that are openly available for reuse are referred to as Public Domain, usually after copyright has expired or does not apply.
Copyright holders can choose to license their works with Creative Commons, which allows for various levels of use and remix restrictions. Visit the Yale University's Arts Library Digital Services guide to copyright page for more details on these topics. Georgetown has an excellent guide to the ins and outs of using images in publications.
Europeana
Search the digital resources (images, texts, sound, and video) of Europe's museums, libraries, and archives.
Each image record includes information on the license type—many images are available for download and use.
more...
The Metropolitan Museum of Art
More than 375,000 high resolution images of public-domain works from the museum's collections. Entries often include links to essays from the Heilbrunn Timeline of Art History on themes, genres, and individual or groups of artists.
The Getty Trust's Open Content Program
Paintings, drawings, manuscripts, photographs, antiquities, sculpture, decorative arts, artists' sketchbooks, watercolors, rare prints from the 16th through the 18th century from the J. Paul Getty Museum and the Getty Research Institute made available for free public use and download.
Go to the Getty Collection to search and limit to Open Content Program. All images found through this program should be credited as "Digital image courtesy of the Getty's Open Content Program".
Wikimedia Commons
Millions of images user-created and contributed images available for use. Keep in mind that you are relying on copyright information provided by the person uploading the image, which may or may not be accurate. Follow links provided to confirm the source of the image and that the uploader had the rights to license.
Les Musées de la Ville de Paris
Over 150,000 free-to-use images of items from Paris museums, including Musée Carnavalet (Histoire de Paris) and Petit Palais (musée des Beaux-arts de la Ville de Paris).
Artstor
Digitized images of art, architecture, and other forms of visual culture from around the world, from museums, archaeology, photo archives, slide collections, and art reference works, dating from B.C.E. to present.
Search for "Images for Academic Publishing" as a keyword to limit to images that can be used in publications.