These images satisfy three criteria:

They are either Wikipedia featured pictures or Commons featured pictures.

They are rather large

They have an aspect ratio of 4:3 or 5:4

They are therefore likely to be good desktop backgrounds (wallpaper).

To check aspect ratio, go to the properties of the image, and then divide its height by its width. If the resulting number is 0.75 or 0.8, then it's the ratio we want. Anything between 0.73 and 0.82 is tolerable.

To put an image in this category, add {{Wallpaper}} to its description page if it is featured on Wikipedia and {{CommonsWallpaper}} if it is only featured on Commons.

See also: Widescreen desktop backgrounds