Hi there, *.IMG files (the actual maps on your GPS) should be stored in the map <GARMIN> either on the GPS or a micro sd card.
On the Dakota 20, GPSMAP 62, Etrex 20/30 and Oregon series you can name them anyname.IMG and thus store each map in a separate file.
So if you make backups of al your maps and give them a meaningfull name you can copy them back and forth to your GPS (or micro sd) at anytime without using Mapsource ore mapinstall. When you use Javawa Device manager you can "manage" the maps when in your GPS and even make them "visible" to Basecamp with a single mouseclick.