I know this is old, but for others with the same issue on iOS:
1. In the Xcode files, find the folder Images.xcassets . This holds the Icons and Splash screen images.
2. Go to https://www.appicon.build/ and build the splash screen images all in one go (plain black image is fine for some)
3. Go to http://appiconmaker.co/ to build all Icon sizes in one go
4. Copy over the images in the relevant folders inside of the Images.xcassets folder