Typically, a wallpaper image may be centered, stretched, or tiled.
When an image is centered, it is placed in the middle of the desktop and is
surrounded by a solid color. When an image is stretched, it is stretched to
cover all of the desktop. Only certain images can be stretched or they look
distorted. An image that is tiled is placed on the desktop much like tiles are
placed in a shower or a tiled floor. Tiling is commonly used for patterns
instead of photos because a pattern is one square image that repeats itself
across and down the screen, effectively forming a single image. (source: SearchwinIT)
