I got it to work. Here's the hack:
Use the resolution of the cheapest iPhone from the most recent lineup - i.e on 2021 it was iPhone SE with a resolution of 750x1334 (width, height).
Make sure to use a filename for the screenshot that is fairly short and contains only letters and numbers.
And finally, the grand jewel of them all - make sure that the extension is capitalized.
filename.PNG
kibernetiko
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Same problem here. After several failed attempts with all sorts of resolutions, the system now returns a "wrong format error".