iPhone instructions

To set a wallpaper on your iPhone, download the wallpaper image and save it to your photos (if it does not download when you click the image or button, touch and hold the wallpaper and save to your photos). Then, open Settings, tap "Wallpaper," then "Add New Wallpaper," and select “Photos” (make sure to select the “All” tab) and choose the wallpaper image, then tap "Add" at the top right and choose to set it as a wallpaper pair or customize the home screen. 

Android Instructions

To set a wallpaper on your Android phone, download the wallpaper (if it does not download when you click the image or button, touch and hold the wallpaper and save to your photos). Then, touch and hold an empty space on the home screen, tap "Wallpaper & style" or "Wallpapers," select your "My photos" and choose the wallpaper image, and then tap "Set wallpaper" to apply it to the home screen, lock screen, or both.