Well, I clearly should have done some homework before starting this thread. I'm obviously confused about a lot of things.
Somehow I got a map from gpsfiledepot (I now suspect maps/view/42) and did something or other with basecamp on a windows machine. I could not succeed in combining it with my US topo on the 60csx for reasons that I now understand, not realizing that I needed mapsource, nor that a version was available from gpsfiledepot that would run on Windows 7. However, somehow I did manage to get the map into basecamp and somehow got it out in img format. How I did this is a complete mystery - perhaps I even succeeded in loading it on the 60csx and then renamed it off of the SD card. Then, turning to a linux machine, I used gmt to combine it with my existing US topo, making a gmapsupp.img, and I've been happy ever since, except for the dots. Getting old sucks, but at least I can still hike. Apologies for all the confusion.