Sorry but I don't get that site. I uploaded my image, all I end up with is this funky image at 1242 x 2208, but Apple now wants 1242 x 2688. I tried others but they all want you to register first.
I finally got XCode 10 installed and was able to run my app in their simulator and generate a correct sized image. But you need a Mac to do that. I though with CodenameOne you didn't need a Mac.
I am so tired of this crap, every time I submit an update to the Apple App Store I have to go through this. I have been working on this all day. Apple sucks! You really need to address this issue.