Are you sure you don't mean img in the file name?
Some cameras create file name like: img001.jpg, img002.jpeg, etc.
Googling .img points to some type of image in AutoCAD?
I find it hard to believe that AutoCAD cannot work with .jpg or tiff.
.JPG is pretty universally accepted by all applications that deal with images.
Who is asking you for an image with the .img file extension and what are they planning to do with it, (use in what application)?
I seem to recognise the .IMG format (as an image format), but can't really remember where it's coming from. But you could try Xnview, which is an excellent picture viewer and converter than can read/write a ridiculous amount (about 500) of different formats.