If you want to share a wallpaper with a friend, tap on the share icon in the bottom right corner of the image.
 

NOTE: The wallpaper images are licensed for personal use ONLY. See the User License in Settings for more information.
 

When you want to use the image as a photo in a custom lock screen (with widgets, etc.) you'll have to save the wallpaper to your camera roll using the share sheet and then set it as you normally would as a custom lock or Home Screen.
 

If you have a monthly or yearly subscription, there are no watermarks on the saved images. The free version will have a Wallaroo graphic at the bottom.