Wherever possible possible the links below lead directly to the open resources held by the various 
institutions. Some of pages have a check box you need to tick to access works that you can download and use.  

Art Institute of Chicago More than 53,000 images of works from the collection that are in the public domain. Some innovative search filters including colour, place and if it includes additional educational resources.

Belvedere, Vienna Half of the Belvedere collection is now accessible online, with more being added including the world-famous group of works by Gustav Klimt and Egon Schiele.
CC BY SA Licence.

Birmingham Museums Trust 2,500 images from across the eight municipal collections looked after by the trust including many Pre Raphaelite items. More items are gradually being added to the list including works by contemporary artists such as Cold War Steve.

British Library Illuminated Manuscripts This portal hosts high resolution images of medieval and Renaissance manuscripts in one of the richest collections in the world. The superb glossary tool provides an excellent visual introduction to understanding the manuscript styles and idioms, materials and contexts. 

British Museum The Museum has made 2 million high-resolution images of items in their collection available for the public to download. CC-BY-NC-SA license.

Cambridge Digital Library Over 23,800 hi-resolution artefacts about discoveries and ideas covering past 2 millennia. "We want our unique and distinctive collections to be openly accessible to as many people as possible where copyright permits," 
Click Download Image’ for Licence information.

Cleveland Art Museum has an Open Access policy allowing public now has the ability to  share, collaborate, remix, and reuse images of many as 30,000 public-domain artworks in its collection. Excellent clean easy to use zoom/download interface complete with extensive descriptions and metadata.

Cooper Hewitt Design Museum in New York has over 210,000 objects in its collection, many of which are available under Creative Commons licences or are in the public Domain.

Cornell University Digital Collection aggregates a broad spectrum digital collections content into a single interface, enabling easier access and discovery of these resources which include historic images, documents maps and photographs.

The Davison Art Center at Wesleyan University holds over 24,000 high -resolution prints, photographs, and other works of art. They include works by Delacroix, Dürer, Goya and Rembrandt.  Download the images in jpeg or tif  formats or view using the high resolution zoom viewer.

Digital Bodleian Over a million images of rare books, manuscripts, and other treasures from the Bodleian Libraries and Oxford college libraries. most are available to download under a CC BY-NC 4.0 licence.

Glasgow School of Art Archives & Collections, a rich resource for the study of art, design, architecture and art education, textiles, historical images of Glasgow and Scotland including a superb collection of objects and architecture by Charles Rennie Mackintosh,  CC BY SA Licence

The Khalili Collection Over the course of five decades, Professor Nasser D. Khalili has assembled eight of the world’s finest art collections. These collections comprise some 35,000 magnificent works of Asian and international art.

The Kunstmuseum Basel Collection has 4,100 photographs in the public domain available to download. The items are arranged by artist in alphabetical order.

LACMA Los Angeles County Museum of Art: Over 20,000 items in the public domain.
Check ‘search public domain only’ box when searching.

The Louvre has put 480,000 works online  - these are being updated on a daily basis. 

Mauritshuis The entire collection of the Mauritshuis is available to download in high resolution. It includes more than 800 works by Dutch and Flemish masters. The Museum website includes other ways to explore the collection interactively, including a virtual Gigapixel Tour’.

Metropolitan Museum of Modern Art includes more than 400,000 Public Domain works. 
Check the Open Access’ box on the ‘Search the Collection’ page

MG&K Collection Online this collection from the Museum für Kunst und Gewerbe Hamburg have over 12800 artefacts available.

National Archive The UK’s National Archive on Flickr, offers a fascinating insight into British cultural history. No known copyright restrictions.

National Gallery of Art in Washington, US, has has over 50,000 open access digital images of artworks. Images of works of art in the Gallery's permanent collection which the Gallery believes to be in the Public Domain are available free of charge for any use, commercial or non-commercial.