how to install map on an etrex vista

Ive downloaded an .img file but didn't find how to use it
Mapsource didn't see the file wherever I put it, seems he didn't recognise .img file.
Is there an archive, so how to extract the files in? I know rar and zip file but not img ?
In which directory must the files be installed to be seen by mapsource?
I didn't find any simple tutorial in the sitsaying where to put files and so one and search didn't give help as a research with .img or img find nothing.

Thanks for the help

.IMG file is already maked with mapsource, it is ready to use map image
rename image file to GMAPSUPP.IMG if it not already

copy it to [x]:\Garmin directory
if it is locked image you must make GMAPSUPP.UNL file with key maked Jetmouse KeyGen
if it unlocked .img you can use it without .UNL file