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Wallpaper (also desktop picture and desktop background) is an image used as a background on a computer screen, usually for the desktop of a graphical user interface. "Wallpaper" is the term used in Microsoft Windows before Windows Vista (where it is called the Desktop "Background"), while Mac OS X calls it a "desktop picture" (previously, the term desktop pattern was used to refer to a small pattern that was repeated to fill the screen).

Formats
Images used as computer wallpaper are usually raster graphics with the same size as the display resolution (for example 1024×768 pixels, or 1280×1024 pixels) in order to fill the whole background.

Dimensions
Screen resolutions that are proportional in a 4:3 ratio, so an image scaled to fit in a different-sized screen will still be the correct shape, although that scaling may impact quality. Common wallpaper resolutions are connected to common desktop resolutions: 800x600, 1024x768, 1280x1024 and 1600x1200.

Users with widescreen (16:9 or 16:10) monitors have different aspect ratio requirements for wallpaper, although images designed for standard (4:3) monitors can often be scaled or cropped to the correct shape without undue loss of quality.

Wallpapers are sometimes available in double-width versions (e.g. 2560×1024) for displaying on multi-monitor computers, where the image appears to fill two monitors.

Some display systems allow unconventionally-proportioned images (1:1, 2:1, or even 1:3) to be scaled without change of proportion, to fit the screen, whether it be 16:9 or 4:3. The image would be sized just large enough that one pair of edges touch the edges of the screen, but not all four, as this would unduly distort the image. In these cases, the system's "default" background color is visible around the other two sides of the image.
Technical aspects

The main problem when enlarging an image to match the desktop resolution is the quality loss that appears during the resize process and the distorted image that appear when aspect ratio of the wallpaper is different than the aspect ratio of the desktop.

These problems can be eliminated by using vector images. However, these images are not very popular and are hard to find.
Image types

PNG and JPEG format are common. Some desktop systems, such as Mac OS (version 8.6 or later), KDE (version 3.4 or later), and GNOME, support vector wallpapers (PICT in Mac and SVG in KDE and GNOME). This has the advantage that a single file may be used for screens of any size, or stretched across several screens, without loss of quality.

Most display systems are capable of specifying a single colour to use as the background in place of a wallpaper, and some (such as KDE or GNOME) allow colour-gradients to be specified. Early versions of Mac OS and Microsoft Windows allowed for small repeating patterns to tile the desktop.

History
Original computer wallpaper pattern, as used in Xerox's Officetalk and Star; actual size.

The first use of a distinguishable background in conjunction with overlapping windows was in an experimental office system, Officetalk, developed in 1975 at Xerox PARC on the Alto. Prior to that, the white backgrounds to overlapping windows (for example, in Smalltalk) could be difficult to distinguish from window interiors. The pattern used in Officetalk produced a 25% gray, using dots two pixels high to avoid flicker on the Alto's interlaced screen. The same pattern was adopted for the Xerox Star.

Apple used a similar gray background for their Lisa and Macintosh. However, since these machines had non-interlaced screens it was possible to use a less noticeable background pattern, formed from a simple 2x2 repeating pattern that gave a 50 percent gray. The introduction of color monitors for personal computers led to non-patterned, single-color backgrounds and then to arbitrary 'wallpapers'. There was a way to get a desktop picture on versions of the Mac OS before Mac OS 7.

Styles
Retro-tech: an Indian Head test card mosaic.
In Personal Use:

Wallpapers can be any type of picture e.g automobiles, models and celebrities, scenery, abstract art, movies, pets, family, symmetry, and personal photos.
In Business Use:

It is common to use corporate logos or plain backgrounds on business computers, as such guidelines are often specified by the company.

It is common to create wallpaper that displays a computer's name when using rack mounted computers through a KVM switch in order to easily identify to which computer the user is connected. Many companies will disable the display settings so the staff can't change the background, however this is often easy to get around for example: using the "set image as background" button in many web browsers.
Dynamic backgrounds

Some operating environments (e.g. KDE, Mac OS X, and Windows 7) allow a number of different wallpapers to be used, and changed from a set of selected images (either in order or shuffled) to display a different wallpaper at different times. In environments where this is not possible off-hand, third-party tools may be able to have this be done.[citation needed]
In Microsoft Windows:
Windows 7's Desktop Slideshow

Windows 98, Windows 2000, Windows Me and Windows XP support Active Desktop, a component that allows dynamic web pages and Adobe Flash to be used as desktop wallpapers. Active Desktop was discontinued on Windows Vista.

On Windows Vista Ultimate, Windows DreamScene can use a video clip or animation as a wallpaper. The animation loops continuously. Windows DreamScene was discontinued on Windows 7.

On Windows 7, Desktop Slideshow allows users to specify a group of images which Windows 7 displays on desktop in sequence for a specified period of time.
In Mac OS X:

Mac OS X has built-in support, via the Desktop & Screen Saver panel in its System Preferences, for cycling through a folder collection of images on a timed interval or when logging in or waking from sleep.

Additionally, OS X has the native ability to run a screen saver on the desktop; in this configuration, the screen saver appears beneath the desktop icons in place of the system wallpaper. However, OS X does not come with a built-in interface to do this; it must be done through Terminal commands or various third-party applications.
With Video Media:

Some media players, like VLC, can redirect video playback to the desktop, allowing any video to be used as a wallpaper. Other such media players on Unix derivatives provide a similar option to output video to the background of X11.
Real-Time:

Programs such as Xplanet and EarthDesk use Internet connections and graphics calculations to change the wallpaper with real data, such as a shadowed view of the earth, the latest cloud or weather map, or various events.

FROM WIKIPEDIA