As discovered by 9to5Mac, the company is giving out a URL on physical cards in retail stores when users trade in old devices for something new, but anyone can visit the site and download the art for themselves regardless of whether or not they've taken advantage of Renew.
Each wallpaper was created by Anthony Burrill, a renowned graphic artist, and was "inspired by nature and people coming together to help the planet." The three pieces of art are called "Nature in Balance," "Nature in Harmony," and "Nature in Us," and come fitted for the screens of the iPad mini, iPad Air, iPad Pro, iPhone 5, iPhone 6/6s, and the iPhone 6/6s Plus.
Users can download the wallpapers on a desktop in a zip file that includes different size versions of each piece of art, but it's easier to navigate to the company's website and save the image directly from the iOS device you wish to use the wallpaper on. That way all that needs to be done is a simple long press to save the image, then visit Settings > Wallpaper > Choose a New Wallpaper, and select one of the new environmental images.