Yes that's correct, *.img files are the sole map data files that are directly usable by Garmin devices - however they are a proprietary Garmin file. A quite different format from the other common files with that same extension name, those IMGs are similar to ISO files. GarminExpress installs maps directly as IMG files to an entitled device.

Raw Garmin mapping data may also be contained in a .GMAP folder which is stored on a computer (aka, a PC Install) and is used by MapInstall to compile and install an IMG file on a device. That 'PC installed' form of map data cannot be seen by a device but can be used by BaseCamp which can then invoke MapInstall to transfer selected map tiles, compiled as an IMG file, to a connected device which can legitimately receive it.

OSM on-line data can be compiled at their website into a Garmin-readable IMG file. To learn how to install an OSM map first start with this guide and its links:
Code:
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Note: I'll soon move these recent posts to a dedicated discussion thread as we're quite off-topic here now.
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