It looks like just a couple of the dynamic Windows 7 themes received updates this past week.  Those updated were one of the original dynamic themes called Bing Desktop and one of the community themes called Flora or Flowers.

The Bing Desktop images should look familiar if you visit the home page as they have been featured there.  The Flora images are from Microsoft’s open call for images which is still ongoing.

Bing Desktop

Halong Bay in Quang Ninh province, Vietnam High angle view of rock formations, Grand Canyon, Grand Canyon National Park, Arizona, USA
Florida manatee (Trichechus manatus latirostris), Crystal River, west-central Florida, USA Beautiful view of the New River Gorge Bridge in West Virginia The largest Steel-Arch bridge in the Western Hemisphere.

Flora (Flowers)

Orange poppy Dew on Catkin strands