I was pleasantly surprised when these walls surfaced on the net these days, like a nice Christmas gift :)) There are built in Google’s Chrome OS as part of a project named Aura. As you can see most of the backgrounds are nature themed and some architecture.
You can choose and download each image whichever feels nice, or I have uploaded four zipped archives .(the images in the archives are random, had to make four due to file size limit on the servers)