The following wallpapers were made for desktops. However, you can certainly use them for phones or tablets! I recommend downloading the highest resolution wallpaper and then crop it. When you set a new background on your device, you're prompted to position it and crop it. View this sample of cropping. If the wallpaper feels too bright or distracting, you can use your device's native photo editing to darken the wallpaper to your liking.
Also, I want to point out that a bunch of the official artworks for this game are vertical posters. So you can hop on over to the Square's Art page to find many options for mobile wallpapers.