I downloaded several maps from this website including Melbourne (Australia) and Hong Kong.
I found a utility called "Sendmap" that will export to a Garmin device via USB and this works with my Quest.
I can view maps on the Quest, select Waypoints etc.. and export the Waypoints, Tracks and Routes back to MapSource on my PC.
The .IMG files only make sense to a Garmin GPS device, you need some more stuff to make it work in MapSource on a PC.
You cannot upload Maps from Quest to MapSource, and I have not worked out how to create the necessary files or Registry values necessary, but there are lots of utilities that claim to do this.