@ catymag,
I've tried two different img files (found on this site) and both were listed as Unlocked.

After reading some advice here I also downloaded GMapTool and imported the img file.
I wasn't exactly sure what I was looking for but the thread said it would indicate in the Details if the file was locked.

I saw no mention of it being locked locked. Here's what I got...

Reading file P:/Garmin/gmapsupp.img.

File: P:/Garmin/gmapsupp.img, length 2010939392
Header: 21.05.2014 21:38:30, DSKIMG, XOR 00, V 0.00, Ms 1, 000-00000-00
Mapset: City North America NT 2015.10