It is possible to use downloaded maps in IMG format with Basecamp, but it requires a couple of steps to create a so called virtual device, which acts like a Garmin device is connected to the PC. Instructions on how to do this can be found at https://www.javawa.nl/virtualdevice.html.
Put the gmapsupp.img file in a Garmin folder on the USB stick (or virtual device), and then start Basecamp. Basecamp will see the map file and index it. Then you can select the map from the Maps menu in order to display it as the active map.
