The download feature doesn't work for me yet either, but there's another cool trick I've been doing for awhile to get Bing backgrounds in 1920x1200 resolution.
If you using firefox, right click on the Bing background and select view background image:
Now it loads the default 1366x768.jpg for you. Just select and change it to 1920x1200.jpg
and tada, full resolution of 1920x1200 goodness: