WikiArt is a great site that is well organized by various searching functions, like artist, genre, medium, subject matter, etc. The images are high quality and are available for download.
The MOMA has a great website filled with images and information. Their educational resources are stellar and well organized.
Metropolitan Museum of Art
The Met is so well organized. You can find almost any object in their collection online with accurate information and great photos.
Hope this is helpful!