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.

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.

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.

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.

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.