Ah, OK, a map img file named gmapsupp.img (or gmapprom.img) can be written directly to a microSD card and inserted into a garmin satnav unit, however usually there can be only one img file so if you already have a microSD card holding an img file you would be losing the existing maps contained in the original img file
It is possible to combine the contents of several img files into a single img file by installing the maps into Garmin Basecamp and then sending all of these maps to the unit in one (bigger) img file