You can change it, to some extent. Once you select a banner image, tap the ... on the appbar at the bottom. It gives you an option to change the image but all it basically does is cycle through all the images you're tagged in. Unfortunately, it doesn't allow you to choose a specific image to display. Should we have that option? Ofcourse. Its a great way to personalize your Facebook experience. But personally I just let it stay at the default Facebook blue. That way the icons are clearly visible. But each person to his own right?