...I found on several websites and after unzipping saved the map as GMAPSUPP.IMG when in fact all one MUST DO is save it as GMAPSUPP as it is already an .img file and the file name is too long.
Sounds like you got bitten by the stupid default behaviour of windows, which is to 'Hide the Extension of known file types'.
So if you have a file which is actually called 'gmapsupp.img', it will just display it as 'gmapsupp'.
If you then re-name the 'gmapsupp' to 'gmapsupp.img', what you've actually done is re-name it to 'gmapsupp.img.img', only windows is hiding this fact from you.
You really need to turn off this behaviour in Windows, and have it show you the full file name, extension and all.
On Win7:
Start -> Control Panel.
Change the 'View' drop-down on the top right to 'Small Icons'.
Folder Options -> View tab -> Un-check 'Hide extensions for known file types'.
'OK' your way out.