The iphone wallpaper size is not universal since each iphone model has a varying screen resolution. Most users though opt to use a slightly larger image than their screen resolution to enable parallax and cropping. The safest method is to use images with high resolution (such as 1242x2688 and so on) in order that they could be scaled among the devices. This is to ensure that there is clarity and that there is no pixelation as the wallpaper is adjusted in order to fit your screen.