.img filename extension that store raw disk images of floppy disks, hard drives, or optical discs.
The .img filename extension is used by disk image files, which contain raw dumps of a magnetic disk or of an optical disc. Since a raw image consists of a sector-by-sector binary copy of the source medium, the actual format of the file contents will depend on the file system of the disk from which the image was created (such as a version of FAT). Raw disk images of optical media (such as CDs and DVDs) contain a raw image of all the tracks in a disc (which can include audio, data and video tracks). In the case of CD-ROMs and DVDs, these images usually include not only the data from each sector, but the control headers and error correction fields for each sector as well.
Since IMG files hold no additional data beyond the disk contents, these files can only be handled by programs that can detect their file systems. For instance, a typical raw disk image of a floppy disk begins with a FAT boot sector, which can be used to identify its file system. Disc images of optical media are usually accompanied by a descriptor file which describes the layout of the disc, and includes information such as track limits which are not stored in the raw image file.
Filename extensions and variants
The .img file extension was originally used for floppy disk raw disk images only. A similar file extension, .ima, is also used to refer to floppy disk image files by some programs. A variant of IMG, called IMZ, consists of a gzipped version of a raw floppy disk image. These files use the .imz file extension, and are commonly found in compressed images of floppy disks created by WinImage.
.img Apple Disk Image
Apple Disk Images are disk image files commonly used by the Mac OS X operating system. When opened, an Apple disk image is "mounted" as a volume within the Finder. Several Apple proprietary disk image formats can be used to create these images, including the Universal Disk Image Format (UDIF) and the New Disk Image Format (NDIF). Apple disk images usually have the .dmg file extension.
Apple disk images allow secure password protection as well as file compression and hence serves both security and file distribution functions; they are most commonly used to distribute software over the Internet.
.dmg, .smi, .img
https://en.wikipedia.org/wiki/Apple_Disk_Image">Apple Disk Image
.img ISO image (based on ISO 9660 file system)
.img Garmin device map format
Garmin .img Format
https://en.wikipedia.org/wiki/Garmin_.img">Garmin .img
http://wiki.openstreetmap.org/wiki/OSM_Map_On_Garmin/IMG_File_Format">IMG File Format - OSM Map on Garmin
https://sourceforge.net/projects/garmin-img/">Garmin IMG Format - A description of the Garmin IMG file format. Includes some experimental/sample code for parsing IMG files, on sourceforge.
http://garmin-img.sourceforge.net/">Garmin IMG Format
The IMG file contains all the info needed to render a map on a Garmin GPS unit, and may support autorouting information for those GPS units that offer navigation (i.e in-car GPS routers and some types of devices intended for hikers). The .img file is basically a hard-disk image (complete with partition table) containing a filesystem which itself contains subfiles that provide the functionality for the Garmin GPS unit. The filesystem seems to be 'flat': that is, it doesn't directly support the concept of subdirectories, however the .GMP subfiles do provide a kind-of subdirectory structure.
http://wiki.openstreetmap.org/wiki/OSM_Map_On_Garmin/IMG_File_Format">OSM Map On Garmin/IMG File Format