Commercial sites:

Bridegeman

Art Resource

Scale Images

Educational Image Archives:

ArtStor

Wikimedia

Google Art Project - https://www.google.com/culturalinstitute/beta/

Art UK ( BBC Your paintings project) - https://artuk.org/

 

Mobile APPs:

Wikiart

Others:

https://www.wikiart.org/

https://www.artsy.net/

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Here is a answer from quora

There are hundreds of online art databases but none of them can be called "comprehensive" nor do any of them look like they will ever achieve comprehensiveness. They fall into several categories:

Fine Art Stock Photo Databases: contain high to very high resolution images for use in publications but very little information about the works and searching and browsing capabilities are limited.  These include Bridgeman, Art Resource, and Scala Images.  These are commercial sites.

Educational Image Archives: contain medium to high resolution images but very little information about the works and searching and browsing capabilities are limited.  These include ArtStor, Wikimedia, the Google Art Project, and the BBC's Your Paintings Project described in an earlier answer.  Some of these are by subscription only (ArtStor) and some are free.  The Google Art Project has great images but a surprising number of problems (artist search by first name only -- e.g., you have to know Cezanne's first name is Paul or you can't find his works, navigating through museum galleries is disappointing as you often find yourself looking down on a painting from the ceiling, information on works is sparse or is stock text from published catalogues).  The Google Project's dependency on museums for images and information is its Achilles Heel and this factor has led to the demise of several projects that were extremely well funded (Getty's AMICO Project, Getty's MESL Project, Getty's Museum Prototype Project) but were dependent on museums for materials.  Museums lack the resources and interest required for success.  The BBC's Your Paintings Project is the most innovative of these though it is limited to paintings in UK public collections but is the best of the lot in terms of organization and ability to provide users with something useful.  

Academic/Hobbyist Sites: contain low to medium resolution images and much more information about the works.  These sites are specific to a single artist or movement.  Though searching and browsing capabilities are limited, they are much more innovative than in the above categories.  These include caravaggio.com, the Picasso Project, and dozens of artist-specific sites.  These are non-commercial archives though they may use pop-up ads or solicitations for donations to support them.  The Picasso Project is quite impressive as it contains over 9000 works by a single artist.  The Caravaggio site incorporates a number of innovative searching and display features.  Most of the other sites in this category contain a few dozen images, minimal information, and no search capabilities.

Mom-and-Pop Commercial Sites:  contain low to medium resolution images, scant or minimal information about the works and no, or extremely limited, searching capabilities.  Most exist only to sell reproductions or "hand painted originals" or objects such as mugs and t-shirts imprinted with images.  These include the "Complete Paintings Of" series of websites and artcyclopedia.com(which features pointers to paintings on museum websites).  Artcyclopedia provides artist bios but its structure of providing an alphabetical list of the museums that hold images of say, Claude Monet, means that the user has to click on each link which takes him/her to a museum's website, locate the image (which is frequently not what the link on the artcylopedia list leads to), look at the image and information, then return to the artcyclopedia.com site and proceed to the next link.  Working one's way through the list of museums that have images of works by Cezanne took several hours.  The inconsistencies of image quality, search and display tools, and widely varying amounts of information on sites made this exercise extremely tedious and frustrating.  

Websites of Individual Museums:  contain low to high resolution images, considerably less information than you would expect from the museums that hold the works, and mediocre searching mechanisms.  Many of the leading art museums in the U.S. are plagued by significant technical and design problems, particularly those based on a commercial product called emuse.  Users are expected to know how to perform sophisticated Boolean searches, assumed to be familiar with expert jargon, and encounter zoom features that frequently do not function, search results that are misleading (a search for "Picasso" may result in 100 hits but only 3 of the hits are works by Picasso -- the other 97 will have the word "Picasso" embedded in text that does not appear on the screen), and images are smaller and of poorer quality than those that can be found by just googling on the artist or going to Wikimedia.  

Other Online Art Databases:  contain very low to medium resolution images, minimal information, and poor searching and browsing capabilities.  These include the Smithsonian's American Art Project, the Library of Congress' American Memory Project, SCRAN in Scotland, the NY Public Library Digital Project, and Foto Marburg.  Some in this category contain over 1,000,000 entries but do not provide the tools needed to enable users to search the materials intelligently.  

No matter how extensive a museum's collection (the Met, the Smithsonian,  the Louvre), it is not comprehensive enough to support meaningful  searches.  These sites provide some pleasurable browsing of eye candy  and little educational content.  There does not seem to be even a single  US site that provides content in a language other than English which is  a disgrace given the large Hispanic population of the US.