You can change the app wallpaper by clicking on "Wallpapers" button from the application menu.
1. Then select the desired image.
2. Once the selected wallpaper is showed click on the button at the bottom right to apply the image as wallpaper of the app or as wallpaper of the iPhone/iPad, but it is necessary Previously to the save the image in photo album.
3. Finally, click on the "Set app Background" button.