Considering the number of completely different IMG file types, it is somewhat difficult to give a precise answer about how to best convert img to kml or how to approach img to kml conversion in general.
But generally speaking, depending on the type of data, you have to pick appropriate software. KML format is text based and describes vector data, so unless your .img file is vector graphics, it will be very difficult to translate it properly. You can try to use one of the online graphics converters for this task.
Exception to this would be attempts to export IMG files from Garmin devices (tracks or routes) to KML format. For this you have to use for Garmin utilities that allow translation of the data to exchange formats, including KML. One of such utilities was cGPSmapper, perhaps you can still find somewhere on the Internet.
Updated: January 3, 2024