On a Windows Vista or Windows 7 PC, from the Start menu, choose Control Panel, then click "Change desktop background" (under the heading "Appearance and Personalization"), then under "How should the picture be positioned" in Vista, choose the centered image, or under "Picture position" in Windows 7, choose "Center." Another way to get to this control panel is to right-click the desktop, choose Personalize, then click "Desktop Background," which is near the top of the window in Vista, and at the bottom in Windows 7. You can even make portrait format pictures look good this way. If the picture you want to use as your desktop background doesn't match your screen resolution - especially if it's less wide or less high - centering may be a better choice than stretching, because the original proportions of the picture are preserved. Some sites offer desktop pictures and wallpaper in non-standard sizes, which may be centered on screen by your system software, while any area not covered by the picture may be filled with a color of your choice (or tiled pattern, for some systems).
Desktop Pictures: How to Center Desktop Pictures and Wallpaper on Screen