I use a LaunchImage set. Click Images.xcassets from the files list. In the next column to the right, you may need to use the "+" button to add AppIcon and LaunchImages image sets. These will have slots for every supported (expected) screen size. If you use these sets, you don't have to worry about encrypting your image files with Apple's indecipherable naming scheme, you just drag each image file to the corresponding slot in the set.
Of course, one still has to know the actual dimensions associated with 1x, 2x, retina4, Retina 4.7, Retina 5.5, etc. I'd certainly prefer Xcode to just show the required dimensions of each image, but it doesn't. Barring that, I'd love a single table from Apple listing all the expected dimensions, but that doesn't seem to exist, either. It looks like Apple has, thankfully, consolidated all of this information in the https://developer.apple.com/ios/human-interface-guidelines/graphics/launch-screen/ under the "Graphics" section. Specifically, as of this writing for devices supported on IOS10:
Device Portrait size Landscape size
iPhone 6s Plus, iPhone 6 Plus 1080px by 1920px 1920px by 1080px
iPhone 6s, iPhone 6 750px by 1334px 1334px by 750px
iPhone SE 640px by 1136px 1136px by 640px
12.9-inch iPad Pro 2048px by 2732px 2732px by 2048px
9.7-inch iPad Pro, iPad Air 2,
iPad mini 4, iPad mini 2 1536px by 2048px 2048px by 1536px
