The process is relatively straightforward, but I want to keep it here so I’ll remember it next time.

The first step is to start from a relatively high-resolution PNG logo:

Once you have the PNG, you need to generate the entire set of icons that are required for various devices/resolution; this can be done at MakeAppIcon.

The generated ZIP file contains an iOS folder, under which there is another folder named AppIcon.appiconset: this needs to be dropped into the Images.xcassets Xcode project folder (and, possibly replace the folder already there).

Finally, for bonus points, you can add the iTunesArtwork-xxx images in the Splash.imageset folder (there may need to be some re-arrangement necessary in Xcode to adjust for sizes, etc. – but it’s all fairly straightforward).