When using launch images (instead of the much easier Launch screen file), you need to provide the properly sized launch image for each device size you wish to support. Once you add the additional launch image, your app should take advantage of the new screen size.
The new iPhone X requires a launch image sized at 1125px × 2436px which is a 3x image for 375pt × 812pt.
Of course if you switch over to using a Launch screen file instead of individual launch images, your app will automatically adapt to all device sizes without any additional work.