Img's filestore is based on FAT system.
[1] Some old GPS units are not able to work with multiple img files so they need to be merged into one file called gmapsupp.img using programs like Mapsource.
[2] The IMG file contains the needed information for rendering 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).
[2] 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.