Although not pretty I use this quick way
1. Create a new LaunchImage and drop any file onto each box
2. Build it , it will fail or have some warnings
3. Click on the "Show Report navigator" and you'll see the sizes/names for each
4. On your image editor resize the image to the proper sizes/names and overwrite to
the respective file as reported in 3. each file is on
/Assets.xcassets/LaunchImage.launchImage
Do the same for icons and don't look back
