Ok, you've decided to go with the Garmin NT CN over the OSM but you've not taken in what i wrote in Post #92: "Be sure to get the PC install data not an already compiled *.img file". You've got an IMG file when you need a .GMAP folder which essentially is a PC Install or MapSource version. It's not possible to install an image file to the PC without firstly reverse-engineering it which loses search function and makes it virtually useless for practical purposes if it's then sent as a reduced area map to your device via MapInstall or MapSource.

So either go with the OSM as earlier discussed, or search for an unlocked .gmap / PC Install / MapSource Version of Garmin City Navigator US & MX or North America. Then once installed you can send to your SP the specific tiles you need for your route.

There's a couple of methods to transfer an already compiled IMG file directly to an older device like your SP2720, one of which is discussed exhaustively in this thread, but there's no way the complete .img file you have will fit on your device because it's available space is limited and far less than what's required even by modern map data for US Lower 48 alone.

Re-read every thing written in my answers to your posts before and ask again for clarification if you don't understand fully. I'm happy to help you with this which is totally do-able, but i cannot supply you with the actual map data due to forum rules and Garmin's EULAs.