From the announcement:
In 2012, the Walters Art Museum, in Baltimore, Maryland, became one of the first American cultural institutions to adopt an open license model for their digitized collections. Using a Creative Commons Attribution-ShareAlike 3.0 license, they released over 18,000 images into the OpenGLAM world. These images were not only available via the Walters website, but, also on Wikimedia Commons.
Since their uploads to Wikimedia, the images have been used in over 4,098 Wikimedia project pages, including Wikipedia articles in over 50 languages. In June 2015, those pages were viewed over 8.4 million times!
While those numbers are impressive, today marks yet another milestone for OpenGLAM: The Walters Art Museum has released their digitized images and metadata under a CC0 license.