500gb take up space on my HD, but don't belong to any visible file or folder
Did you emptied the trash after removing the image file (if it was removed in Finder).
Check in About this MAC > Storage, if this 500 GB is not used by Backups. If it is, that means that it might be local snapshot of time Machine backup including this file. See http://support.apple.com/en-us/ht4878