I suspect many Windows users haven't realised that they can download Windows themes and wallpapers from Microsoft's online Personalization Gallery. The gallery has recently been given a Windows 8-inspired update, including the ability to preview the images included in themes before you download them and a stack of new Windows 8 panoramic themes.
You can filter the gallery to show themes solely for Windows 7 or Windows 8/RT (Windows 7 themes work in Windows 8, but many Windows 8 themes only work on the newest operating system). Alternatively, you can browse by topic or just choose themes which regularly change images using dynamic RSS feeds. Each theme shows details of what elements it changes (themes can include colours and sounds as well as wallpaper), and can be downloaded direct from the site.
The Windows Personalization Gallery is new and improved! [Windows Experience Blog]