The Metropolitan Museum of Art has just placed 200,000 CC0 images of its art and others items in its collection. So far the site is holding up nicely in terms of managing the traffic.
The test files I had from it were large, at around 4000px, and were also crisply photographed. The site’s search works cleanly and a single click downloads the largest file size as a .jpg. The only thing I’d change is to allow a Web browser’s Back button to work with the site’s search results.