Remember that you have a phone with a 5" screen and yet it has a 1920x1080 screen resolution. You have to apply HD images because of this. If you download a 640x480 pic, and then crop it to fit as a wallpaper, it will be blurry. Android stock launcher has always sucked when it comes to setting wallpaper unless you do some fine tuning. Download Nova launcher and use that to set your wallpapers. It gives you the option to set a stationary portrait wallpaper, instead of the scrolling wallpaper that looks sloppy. Setting a wallpaper with Google Photo should do the same if you don't want to use a different launcher. In any case, the higher resolution pics will yield better results.