The official Windows Themes directory at windows.microsoft.com hosts some beautiful themes for your Windows desktop. Some of the newer themes – like these dual-screen panoramic images – are however compatible only with Windows 8 machines.
If you aren’t planning to make the upgrade to Windows 8, the good news is that you can still extract the wallpaper backgrounds of these Windows 8 specific themes and use them on older Windows PCs.
The Windows theme files have a .themepack extension but the Windows 8 specific themes are distributed as .deskthemepack files. Internally, this is just another archive format that you can easily open using the free 7-zip utility.
Install the 7-zip utility and then download any of the .deskthemepack files from the Windows Themes gallery. Press F2 to rename the theme file and change its extension to .7z (you can just append .7z to the file name). Now you can right-click this files and extract the included wallpapers to your desktop.
Now you can either use these background individually or put them in one folder and then open Desktop Background under the Control Panel. Browse for the folder containing these pictures and select the ones that you would like to use on your desktop.