Some sites offer desktop pictures and wallpaper in non-standard sizes, which may be resized by your system software (typically called "stretch" on PC or "fill screen" on older Mac systems) to match standard screen resolutions (display areas). In the past, when resizing a picture with Stretch or Fill Screen, the quality of the picture could be slightly compromised, especially if the subject of the picture had clearly defined edges, which could become slightly jagged. Even then, many pictures were not affected much, as long as the size was a fairly close match. Newer computers typically use high-quality resizing software, so pictures that are larger than your screen resolution will look as good "squeezed" as centered. (If the picture is substantially smaller than your screen resolution, though, there's no software that will stretch it without some loss of quality.)
How to Stretch to Fill Screen on a PC:
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 stretched image, or under "Picture position" in Windows 7, choose "Stretch." 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. (Either way the path is Start > Control Panel > Appearance and Personalization > Personalization > Desktop Background.) Windows 7 also has an option to "Fill," which will crop some of the picture if it doesn't have the same proportions (aspect ratio) as the screen, while "Stretch" will change the proportions of the picture to fill the screen without cropping.
On a Windows XP PC, from the Start menu, select Control Panel: Appearance and Themes: Display: Desktop, then choose Position: Stretch (right-clicking the desktop, and choosing Properties: Desktop is another way to get to this control panel).
For earlier versions of Windows, from the Start menu, select Settings: Control Panel: Display: Background, then choose Stretch (right-clicking the desktop, and choosing Properties: Background is another way to get to this control panel).
How to Stretch to Fill Screen on a Mac:
On a Mac (operating systems prior to Mac OS X), from the Apple menu, select Control Panels: Appearance: Desktop, or Control Panels: Desktop Pictures: Picture, then choose Fill Screen (control-clicking the desktop, and choosing Change Desktop Background... from the pop-up menu, is another way to get to the same control panels).
For recent versions of Mac OS X (earlier versions of Mac OS X lack options for positioning and resizing), from the Apple menu, select System Preferences: Desktop & Screen Saver: Desktop (control-clicking the desktop, then choosing Change Desktop Background... from the pop-up menu will also get you to this control panel), then choose "Fill screen" or "Stretch to fill screen," the difference between these options being that "Fill screen" will crop some of the picture if it doesn't have the same proportions (aspect ratio) as the screen, while "Stretch to fill screen" will resize the picture to match the proportions of the screen.
Also see How to Center Desktop Pictures and Wallpaper on Screen and Saving and Displaying Desktop Pictures.