Standard ISO image files contain the contents of an entire disc, including the boot sector in a single file. These multi-platform files interoperate with many different operating systems and applications. IMG files are used on Apple's Mac OS X along with the DMG format, and are similar to ISO files in that they contain multiple files within a single file. You can convert an ISO image to the IMG format in a Terminal session using Apple's built-in hdiutil command.
Open a Terminal window from the Utilities folder.
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Navigate to the folder containing the ISO image. For example, type cd Desktop and then press Return.
Type hdiutil convert source.iso -format UDRW -o destination.img and then press Return. Replace "source" and "destination" with their actual values.