If the image file is truly unlocked, you need do no more than copy it into a Garmin folder on your SD card. Rename it to gmapsupp.img if necessary (gmapsup1.img will also work if you've updated the fw).

To check it's unlock status, open the .img in [Only registered and activated users can see links. ] and press 'Info'. You should not see an 'L' after 30 under the heading 'prio'.