Not true. Apple's Disk Copy used .img as the extension for disc image files since System 6, if not before then. It remained Apple's extension for disc image files until well after the release of MacOS X when it was superseded by .dmg.
That said, .img has been used by many other developers for many other things. I have seen it used by a scanning electron microscope as the extension for its digital micrograph files. (I presume that the SEM saves micrographs in the GEM bit image format.) The takeaway message is that the meaning of .img is determined by the source of the file. If you don't know where your file came from, then you can flail around forever trying to figure it out.