While viewing a photo, tap the Menu key , then tap Set image
as. Choose the wallpaper location by tapping Home screen wallpaper or Lock screen wallpaper. Tap Crop photo or Crop picture, then tap Always or Just once. After you've cropped the image, tap OK (Gallery
app) or Save (Photos app).
Double-tapping an option (Crop photo or Crop picture) automatically starts the cropping function.