There are many image databases on the internet. In many cases the images within these databases will not be retrieved by search engines.
Image databases come in all shapes and sizes: ranging from large databases of professional institutions like archives or museums to small image collections on individuals' websites. The quality of the images differs enormously.
Image databases can be found using search engines or portalsites.
For more information about Artstor Public Collections consult the libguide created by Artstor