Yes it is. If you go to wikipedia and look up Disk Image, and scroll down to 'Apple disc Image' you'll find this:
Originally Posted by "Wikipedia.org
Mac OS X natively supports .dmg, .toast, Windows .img .mds images if renamed .iso...
So a .img file created on a windows computer will work on a mac. And vice versa. You do not have to first convert it to an ISO image and add the .cdr tag (if opening on a mac) or remove the .cdr (if opening on a pc).
Hope this helps