IPhone Wallpaper is a term that refers, generally, to full screen images viewed on certain system elements of the iPhone or iPod touch. But, depending on the scenario, it has different meanings and implications. While technically wallpaper is only the image that is displayed under the app icons on the homescreen, it is also often used to refer to the lock screen image on iOS devices.
iPhone Wallpaper on iOS 4 and Newer
For iPhones and other iOS devices that can run iOS 4 and newer, the wallpaper and lock screen images can be customized, much like what has long been available on jailbroken phones. On iOS 4, users can select a lockscreen image or homescreen image to place under their icons. The image used for both the wallpaper and lock screen can be the same or different, based on user preferences.
iPhone Wallpaper on Older Devices
On iPhones and iPod touches that can't run iOS 4, the only wallpaper-type image that users can control is the lock screen, the image that appears when a sleeping iPhone is woken up and displays the unlock slider, time, and a background image. This image appears when the lock or home button of a sleeping iPhone or iPod touch is clicked.
iPhone Wallpaper on Jailbroken iPhones
Prior to iOS 4, users of jailbroken iPhones or iPod touches could customize both their lock screen image and wallpaper. To add wallpaper that way, the user had to jailbreak their iPhone or iPod touch (which can be risky) and install specific applications or content. Since iOS 4 has added these features, it is no longer necessary to jailbreak to customize iPhone wallpaper.
Other iPod Wallpapers
Some third-party programs allow users to create and install themes onto older, non-iOS iPods. In that case, wallpaper can also refer to an image or color scheme inserted behind the iPod menu.