Mob.org has a huge selection of free phone wallpapers suitable for use with a wide range of smartphones. Other good sources of free phone wallpapers include Android Central and Zedge.
While Android Central advertises itself as a website for Android users, the wallpapers on the website are suitable for use on any smartphone, including iOS and Windows Phone devices. In addition to providing free phone wallpapers, Zedge also offers free mobile and tablet themes and ringtones. Zedge offers a free app for Android and iOS users that makes it easy for owners of those smartphones to download wallpapers and activate them on their devices.