| See the Images Research & copyright Duke University's image research services page provides easy links to 1) image databases which Duke either subscribes to or recommends, 2) scanning standards, 3) finding scholarly/legal images, and 4) outside image sources. | 
The scholarly Encylcopedias of art history are all heavily illustrated with reliable images.
General
Encyclopedia of world art. 18 vols.  New York, McGraw-Hill 1959-1987.  Lilly  N31 .E4833 1960 
   Major English-language essay-length scholarly encyclopedia.  Articles are signed by major art historians.  Half of every volume is plates.  
Ancient Art
Enciclopedia dell’arte antica, classica e orientale.   Rome, Istituto della Enciclopedia italiana. 7 vols.  1958-1966.  Lilly  N31 .E48 1958 
   
Medieval
Enciclopedia dell’arte medievale.  Rome, Istituto della Enciclopedia italiana, 1991-2002.  12 vols.   Lilly N5965 .E52 1991   
Other Languages
Spanish
Pijoán, José, general editor. Summa artis, historia general del arte.  Madrid : Espasa-Calpe, 1931- .  Lilly N5300 .C72 1931 v.1   
German
Propyläen Kunstgeschichte. 18 vols. Berlin: Propyläen Verlag, 1966-1974.  Lilly N5300 .P76
