I've got a problem: 500gb take up space on my HD, but don't belong to any visible file or folder. This may have something to do with a disk image of 500gb I created and deleted some time ago. Is there anything that can be done with this except backing up the disk, reformatting and copying the contents back in? (That will take a lot of time.) The format of the disk is Mac OS extended, encrypted, journaled.
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
I recommend you scan your Disk with 'Daisydisk' and locate the files that are eating up your Diskspace.