Where did you get your free maps from? .img extension is not an image file but an "image" of data. Think of it as another form of compression like WinZip.
If I may suggest go over to: https://www.gpsfiledepot.com/ Fine the state you want Topo maps for and download the maps from there, with their installer. It will load the maps you want into Garmin's Basecamp program and then you can view them and work with them in Basecamp and then upload them to your GPS if you like them. There is a lot of good data and tutorials on that site.
I have done the converting images from online maps sources and similar and loaded them into Google Earth then Basecamp and then into my GPSMAP 62s and was never satisfied with the results. Getting them scaled and calibrated accurately was difficult and they never looked that good, especially when zoomed way in on your GPS unit. The topo maps from the gpsfiledepot are excellent. The data scales with the zoom level so even if you are zoomed way in the data is presented in a readable format.
If you do have an actual image file you are trying to turn into a map follow this link to Garmin's instructions on how to do it: http://www8.garmin.com/outdoor/custommaps_instruct.pdf. You will need to get a copy of Google Earth. Search for Google Earth Pro. It use to cost money but Google made if free about a year ago and it has some nice upgraded over the old free version.