I've found online art galleries both entertaining for browsing and enlightening when looking for information about historic periods.
I've listed museums, commercial galleries and auction houses that are easy to browse.
MUSEUM OF FINE ARTS - BOSTON The collections page
WORLD IMAGES from California State University. Paintings, photographs, interiors, etc.
VINTAGE IMAGES Free vintage pictures, photos, and illustrations
LARGE LIST OF FREE VINTAGE IMAGE WEBSITES
ARC you can search by artist, or browse the alphabetical list page by page.
ART UNFRAMED over 23,000 paintings
OLDMASTERPRINT.COM etchings, engravings and drawings from the 15th,16th and 17th centuries
ANTIQUE MAPS AND PRINTS
ROYAL ACADEMY OF ARTS
SCHWARTZ GALLERY a fine art gallery in Philadelphia
SHANNON'S fine art auctioneers. This link will take you to the archives page, pick a sale date, then view.
TATE MUSEUM a link to the search page
WEB GALLERY OF ART