Bing has awesome background images that are very pleasing to the eye as well as to the desktop 🙂
Here are two comprehensive sites that have all the Bing images and themes that can be downloaded and used :
Best Of Bing Windows 7 theme pack collection
In case you are not able to download Bing images from the Bing image archives using Firefox, simply open the site in Internet Explorer, right click on desired picture after opening it in full view and select Save Picture As to download it.
Cheers.
