Using 'ionic cordova resources' will generate all-size splashs and icons for selected platforms automatically.
But this method depends on cloud service of ionic, so that when you can't connect to network, this method would be failed.
There is a awesome tool can deal with these problem: cordova-resgen.
This tool base on cordova-splash and cordova-icon, using graphicsmagic to cut pictures.
brew install graphicsmagick