Sony's launcher lets you choose different images as wallpaper for home screen and lock screen separately. Set and change lock screen wallpaper on Xperia Z1 by doing as follows:
1. Open Settings application
2. Under Device settings, tap Personalization.
3. Open Lock Screen.
4. Now simply select your favorite picture to set it as lock screen wallpaper on Xperia Z1
5. You may Crop the selected picture