Hey Chief, you are right that it looks for certain map images like gmapsupp.img, but it is possible that other names are compatible, even a gmapsup1.img. You might play around with this for future use and see if you can get more than one map and it recognize both? I would make sure you are upgraded to the latest firmware for your GPS too. I think gmapsupp & gmapsup1 are used to read maps from an SD card.
These names work on some other GPS's (gmapprom.img, gmapprom1.img, gmapsupp.img, gmapsup1.img, gmapoem.img)
I found this thread that might help you - http://forum.openstreetmap.org/viewtopic.php?pid=301968