I use Nova Prime on my 6P. Here's how to change lock screen wallpaper: Long press on Home Screen space to reveal desktop screens config with "Wallpapers, Widgets, Settings" at bottom. Select Wallpapers. Swipe the choices at bottom to the left and select "Apps & Themes". Select the Google Now Launcher (it's the only one I see). Note: your launcher has not changed and this only uses G Now Launcher to change the wallpapers. Select new wallpaper image from your photos or from whatever G Now offers you by swiping choices to left. After you see the check mark on the image, at the top left, press "Set Wallpaper". A menu will pop up allowing you to select the chosen image for your Home screen, Lock screen, or both. Done.