As of this writing, The Athenaeum lists their online collection of art images at 43,339 (with 14 added in the last seven days), making it one one the largest art resources on the web, perhaps second only to the Art Renewal Center.
The Athenaeum is one of my favorite online sources of images from art history; they frequently have good selections of a given artist’s work, reproduced large enough to enjoy and with well balanced color (which can be a problem on some art image repositories).
In the lists for individual artists, be aware that there are frequently multiple pages of thumbnails, linked from small numbers at the top of the list. You can sort these lists by title, date and medium and toggle the order of each.
Click through the thumbnail or title link to the detail page for the work, and click on the image again for the large reproduction.
You can also browse a museum list; these lists can be sorted by title, artist or date. In the museum listing details click on “Artworks at this museum” at the top to see works in the Athenaeum archive from that museum’s collections.
This can be a fascinating way to browse, in that it produces an interesting mix of artists and styles.
The above images, for example, are all from the collection of The Pennsylvania Academy of the Fine Arts (from top: Edmund Tarbell, Raphaelle Peale, Thomas Eakins [no longer in the collection, alas], Cecilia Beaux, Winslow Homer and Theodore Robinson).