BaseCamp will read .img files from cards and devices that are created by Garmin and/or licensed map creators. It will not read user-created .img files. .img files is not a supported method for exchanging maps. Since I don't know the source of your maps, I don't know how to advise you to proceed.
Garmin does not support third party maps in the .img format, as azdrmn has said. Nevertheless, Garmin-compatible maps can be created with freeware and shareware tools. You can't "open" .img files in Basecamp, they need to be installed.
See the tutorials at gpsfiledepot.com. That would be a more appropriate place to discuss this. You will need access to a Windows machine to build proper mapset.
BaseCamp will read maps that are in a .gmap bundle. If you can convert your maps to that format and put them in your $(HOME)/Library/Application Support/Garmin/Maps directory, BaseCamp should be able to find and display them.