Sorry, but we couldn't determine the size of the image. Maybe your image is corrupted?
I found this bug report, which has been closed last summer. However I just reproduced the exact error message on 1.6.4 stable, when trying to upload an SVG image - our site logo, which currently serve from an external source.
Ubuntu file manager (Nautilus) has no issues reporting the image size 692x187.
We weren’t sending the content-type to S3 when storing optimized images. It was fine for images because browsers detect it from the extension (or the first few bytes) but for SVG, it was not properly interpreted by all the browsers).