Start Here First:
The Bridgeman Art Library is the image clearing house for many museums, galleries and private collections. Even museums that have their own image library links may have images in the Bridgeman Library, so check there first. For example, the Holbein painting of Anne of Cleves owned by the Louvre can also be licensed through Bridgeman.
Website: http://www.bridgeman.co.uk/
People, collections, etc. with no offical websites or representatives are not listed
Archbishop of Canterbury
Website: http://www.archbishopofcanterbury.org/
See Lambeth Palace Library (below) for image licensing information
Ashmolean Museum, Oxford
Website: http://www.ashmolean.org/
Reproduction Fees for Photographic Images: http://www.ashmolean.org/services/photographicservices/
Bibliotheque Nationale de France
Website: http://www.bnf.fr/
Birmingham Art Gallery
Website: http://www.bmag.org.uk/
Picture Library: http://www.bmag.org.uk/index.php?type=perform&maincat=1&subcat=44
Bodleian Library, Oxford
Website: http://www.bodley.ox.ac.uk/
Reproduction information: http://www.bodley.ox.ac.uk/dept/pubs/copyright/pubuse.htm
British Library
Website: http://www.bl.uk/
Copyright Statement (with contact info for reproductions): http://www.bl.uk/copyrightstatement.html
British Museum
Website: http://www.thebritishmuseum.ac.uk/
Compass collections database: http://www.thebritishmuseum.ac.uk/compass/index.html
Cambridge University Library
Website: http://www.lib.cam.ac.uk/
Christ Church College, Oxford
Website: http://www.chch.ox.ac.uk/
Collection du Musée Granvelle - Musée du Temps, Besançon
Website: http://www.besancon.com/
College of Arms
Website: http://www.college-of-arms.gov.uk/
Courtauld Institute of Art, University of London
Website: http://www.courtauld.ac.uk/
Photographic services contact: http://www.courtauld.ac.uk/photosurvey/contact.htm
Fitzwilliam Museum, Cambridge
Website: http://www.fitzmuseum.cam.ac.uk/
Photographic Department: http://www.fitzmuseum.cam.ac.uk/dept/photography/
Frick Collection, New York
Website: http://www.frick.org/
Email contact for reproduction rights is on the front page of their website
Grimsthorpe and Drummond Castle
Website: http://www.grimsthorpe.co.uk/
Hever Castle
Website: http://www.hever-castle.co.uk/
Media contact: http://www.hever-castle.co.uk/media/default.asp
Isabella Stewart Gardner Museum, Boston
Website: http://www.gardnermuseum.org/
Reproduction rights contact information: http://www.gardnermuseum.org/information/rights.asp
King's College, Cambridge
Website: http://www.kings.cam.ac.uk/
Kunsthistorisches Museum, Vienna
Website: http://www.khm.at/homeE/homeE.html
Reproduction information: http://www.khm.at/system2E.html?/homeE/reproduction.html
Lambeth Palace Library
Website: http://www.lambethpalacelibrary.org/
Reprographic services: http://www.lambethpalacelibrary.org/reprographics/reprographicservices.html
Louvre, Paris
(also listed as Musée du Louvre)
Website: http://www.louvre.fr/
RMN Photographic Agency: http://www.photo.rmn.fr/
Mansell Collection
(now part of the Time-Life Collection and/or Getty Images collection?)
Website: http://www.timelifepictures.com
Rights & Clearance: http://creative.gettyimages.com/source/services/rights.aspx
Musee Calvet
Website: http://www.avignon.fr/en/musees/calveten.php
Museum of London
Website: http://www.museumoflondon.org.uk/English/
Collections enquiries: http://www.museumoflondon.org.uk/English/Collections/AccessEnquiries/ContactDetails.htm
National Gallery, London
Website: http://www.nationalgallery.org.uk/
Picture Library: http://www.nationalgalleryimages.co.uk/
National Gallery of Art, Washingon D.C.
Website: http://www.nga.gov/
Visual services: http://www.nga.gov/resources/divsdesc.htm
National Gallery of Ireland
Website: http://www.nationalgallery.ie/
Rights and Reproductions: http://www.nationalgallery.ie/html/rights.html
National Maritime Museum, Greenwich
National Museum of Wales
Website: http://www.museumwales.ac.uk/en/home/
Copyright information: http://www.museumwales.ac.uk/en/copyright/
National Portrait Gallery, London
Website: http://www.npg.org.uk
Picture Library: http://www.npg.org.uk/live/plmenu.asp
National Trust
Website: http://www.nationaltrust.org.uk/main/
Offentliche Kunstammlung Basel
Website: http://www.kunstmuseumbasel.ch/de/
Parham House and Gardens
Website: http://www.parhaminsussex.co.uk/
Pitkin Guides
Website: http://www.britguides.com/
Prado Museum, Madrid
Website: http://museoprado.mcu.es/home.html
Copyright and contacts: http://museoprado.mcu.es/icopy.html
Public Record Office (Now the National Archives)
Website: http://www.nationalarchives.gov.uk/
Copyright: http://www.nationalarchives.gov.uk/legal/copyright.htm
Queen's College, Cambridge
Website: http://www.queens.cam.ac.uk/
Radio Times Hulton Picture Library (also Hulton Archive)
Now at Getty Images
Website: http://www.gettyimages.com/
Royal Armouries
Website: http://www.royalarmouries.org/
Film & Image Enquiries: http://www.royalarmouries.org/extsite/view.jsp?sectionId=2450
Royal Collection (all locations)
(including the Royal Library)
Website: http://www.royalcollection.org.uk/
Contacts: http://www.royalcollection.org.uk/default.asp?action=article&ID=73
Schlossmuseum, Weimar
Website: http://www.swkk.de/en/index.html
Scottish National Portrait Gallery
(part of the National Galleries of Scotland)
Website: http://www.natgalscot.ac.uk/
Picture Library: http://www.natgalscot.ac.uk/index.asp?centre=html/8-pubs/8_pubsFS.asp?status=picture
Society of Antiquaries
Website: http://www.sal.org.uk/
St. John's College, Oxford
Website: http://www.sjc.ox.ac.uk/
Sudeley Castle
Website: http://www.sudeleycastle.co.uk/
Thyssen-Bornemisza, Madrid
Website: http://www.museothyssen.org/
Victoria and Albert Museum
Website: http://www.vam.ac.uk/
V&A Images: http://www.vandaimages.com/
Viscount De L'Isle
(collections at Penshurst Place)
Website: http://www.penshurstplace.com/
Contact information: http://www.penshurstplace.co.uk/contact-us.html
Walker Art Museum, Liverpool
Website: http://www.liverpoolmuseums.org.uk/walker/
Image licensing contact: audrey.hall@liverpoolmuseums.org.uk
Wallace Collection
Website: http://www.wallacecollection.org/
Copyright and Reproduction: http://www.wallacecollection.org/newsite/public/templates/tmpl_generic_1_right_caption_sub.php?pageid=262&openmenu=1_32_0
Weidenfeld and Nicholson
(part of the Orion Publishing Group)
Website: http://www.orionbooks.co.uk/
Westminster Abbey
(also listed as Dean and Chapter of Westminster)
Website: http://www.westminster-abbey.org/
Contacts page: http://www.westminster-abbey.org/contact/index.html
Weston Park Foundation
(also listed as Trustees of the Weston Park Foundation)
Weston Park website: http://www.weston-park.com/
Woburn Abbey
(part of the Bedford Estates)