File with an .IMG extension contains a disc image which could be mounted in Mac OS file system (up to 9th version), as a separate hard drive or an optical drive. IMG files sometimes can be opened by changing their extension into ISO.

IMG files were stored on hard drive, however, when mounted, they were displayed as separate discs or CDs/DVDs. Since Mac OS X system, this format was replaced by the DMG format, although it still can be opened.