Bing is known for its cool photos that serve as the backdrop for the homepage. They change every day, and are mostly unrelated to any current news, anniversary, or holiday (with some exceptions). For the most part, they're simply cool images that spruce up the search box. Bing says that users have been asking for a quick, easy way to download the images so they can use them for their personal wallpaper.
Now, that's a reality. Starting today (in the U.S.), users can click on a download icon at the bottom right-hand side of the image and a high-res version of the image will now be theirs to use in whatever way they please. Although the downloaded image removes the Bing search box and other noise, a Bing watermark is still present at the bottom right corner.
In order to download the image, Bing says that you have to connect with Facebook. Other than pushing the Bing/Facebook partnership, there's really no other reason for this. It's not pushing the activity to your Timeline or anything.
Bing has also put another button in that bottom right-hand corner, an intersecting arrows icon that lets you take the image fullscreen (which gets rid of everything except for the actual search bar).