Often times it is easier to leave your rig with the default desktop settings. Both Windows and Mac OS X have a nice default setup and although they are recognizable, sometimes you want more.
Custom desktop wallpapers may be found all over the web and almost all of them for free download. I want to present a collection of night wallpapers focused on landscapes and cities. All of these examples are from InterfaceLift which publishes over 10 image dimensions & sizes for desktops and mobile screens.