You can either have it display from a Google Photos photo album, and get the version of the interface which constantly displays ads to you, or you can switch the interface to "apps only" mode which will only show you one big ad on the home screen. In "apps only" mode, the thing won't display your photos, either as a screensaver or anything else. You still need to be logged into your Google account, of course; as far as I can tell, not displaying photos is just a way of punishing you for trying to reduce the ads you see.