Seldom
4
It looks like mkgmapGui doesn’t have way to enter command line options and creates the IMG files you found. To do what you need to do you’ll probably need to download OSM data directly from OSM and then run mkgmap on your data from the command line.
It takes a while to figure it out, but here’s the place to start:
http://wiki.openstreetmap.org/wiki/Mkgmap