*.img file supporting software name
A little more information would be helpful as there are a number of apps that use .img files. e.g. my Netgear router uses .img files to update its firmware.
If this is a large file (CD or DVD sized) then it is likely to be an image of a CD or DVD; you can try mounting it with Virtual Clonedrive (free) to see what’s inside it or writing it to a blank disk with ImgBurn, also free.