I had the same problem, using Chrome on MacBook Pro (2017 model, running Big Sur). Yesterday tried to upload my png formatted screenshots that have correct resolution, but got always this same error. Tried several times with both "Choose file" link and drag-and-drop, but no luck. Today refreshed the page - needed to login again - uploaded all four images (using "Choose file", exactly the same images as yesterday) without any problems. Go figure...