Same problem. I tried to upload new screenshots yesterday evening and only some of them were uploaded successfully.
What I've tried: Removing and re-uploading only the problematic images
Removing and re-uploading all images for all languages and devices
Tried using jgp and png
Tried the steps above on Safari and Chrome
Finally I contacted Apple and now I'm waiting for their response.
saturninoc
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New York, NY, United States