Given that you have now succeeded (?) - could you submit a PR, even if only to the docs?
A bit late I know, but I just used this successfully to get all images set in my iOS app.
Really easy to use, and also gives the required icons and splashes lists to paste straight into your mobile-config.js
i think your problem might be that you are using an outdated version of Xcode and the Application Loader for your submission.
Make sure that you are upgraded to the newest non-beta version of Xcode and lmk and see what happens may be you are able to publish your ios app.