As with the other answers I use an Asset Catalog and simply drop files into the LaunchImage slots in the Asset Catalog in Xcode. Then I find my app in the Products Group of the Xcode Navigator (tall pane on the left) and Show in Finder. Any copy of your app package will do. Then I Show Package Contents on my app and all the LaunchImage*.png files are there. I just added a portrait launch image for the iPhone X. To see the names and sizes of the immages I navigated to the package in Terminal and ran the file command on the LaunchImage*.png files.
% file LaunchImage*.png
LaunchImage-1100-2436h@3x.png: PNG image data, 1125 x 2436, 8-bit/color RGB, non-interlaced
LaunchImage-700-568h@2x.png: PNG image data, 640 x 1136, 8-bit/color RGB, non-interlaced
LaunchImage-700-Landscape@2x~ipad.png: PNG image data, 2048 x 1536, 8-bit/color RGB, non-interlaced
LaunchImage-700-Landscape~ipad.png: PNG image data, 1024 x 768, 8-bit/color RGB, non-interlaced
LaunchImage-700-Portrait@2x~ipad.png: PNG image data, 1536 x 2048, 8-bit/color RGB, non-interlaced
LaunchImage-700-Portrait~ipad.png: PNG image data, 768 x 1024, 8-bit/color RGB, non-interlaced
LaunchImage-700@2x.png: PNG image data, 640 x 960, 8-bit/color RGB, non-interlaced
LaunchImage-800-667h@2x.png: PNG image data, 750 x 1334, 8-bit/color RGB, non-interlaced
LaunchImage-800-Portrait-736h@3x.png: PNG image data, 1242 x 2208, 8-bit/color RGB, non-interlaced
