We’ve already showed you how to create and share a theme in Windows 7, and also how to add additional images into the existing themes. Also be sure to check out the awesome Windows 7 theme from How-to-Geek.
InterfaceLIFT is an other place you can download high resolution wallpapers for your Windows 7, Vista, XP etc. They also have an iPhone application you can download, as well as themes for Windows XP (for sale) and icons.
The nice thing is that you can choose the screen resolution and download it instantly. They even have support for Widescreen, Fullscreen, HDTV, Dual Monitors, Triple Monitors and Mobile Devices.
Check it out, its cool. You can search it by tags as well.