Wallpapers can be any type of image, but most users prefer a clean background that is not too "busy." The simpler the wallpaper image is, the easier it is to view icons and windows on the desktop. Some people prefer realistic backgrounds, while others opt for computer-generated images.
Besides choosing a wallpaper style, it helps to choose an image that matches the resolution of your display. For example, if you select an HD (1920x1080) pi for a 4K display, it will appear blurry. Additionally, if the aspect ratio does not match your display, part of the image may get cut off when applied as a wallpaper on your device.
NOTE: A wallpaper is different than a screen saver, which is an animation that appears when a device is idle.