ArchNet Digital Library: Images of contemporary buildings, historical monuments & general views with a special focus on the Islamic world.
Artstor: Digital Library: More than a million images of art and architecture from art museums and galleries
Getty Research Institute: Images ranging from the 15th century to the present, including books, photographs, manuscripts, and works on paper.
Google image search: Google Images. The most comprehensive image search on the web.
Oxford Art Online: Search a range of Oxford’s art reference works, including images
Pixabay: A vibrant community of creatives, sharing royalty-free images, videos, audio and other media