1) Were these images resized by 4images?
2) How were these images uploaded: through members upload form, ACP (Admin Control Panel) or through FTP?
Somehow I think there is something wrong with your server, because I've tryed download that image (open directly in the browser) several times, and 4 times out of 5 your server returned different size of the image content...
Maybe updating nginx to lattest version (0.3.27) would solve the problem...