To avoid any misunderstandings:
.img files are NOT disk images *. As stated above, in this case they are just compressed images of the Netgear operating software. You download the image file to your Mac, then install it onto the Netgear router using the router's web interface - typically at the url: http://192.168.0.1/. You can do this using Safari or Firefox or any other web browser you want to run on your Mac.
Log in as "admin". If you haven't changed the password (you should, you know!), it will still be the default one listed on the router's label.
Click "Upgrade Router" in the left hand column, and select the .img file you downloaded.
* .dmg files are disk images on a Mac.
Demon since forever, HomeOffice MaxDSL since 2006 via Netgear DG834G router
Mac Pro 2.66GHz 8GBytes / MBP 13" - Snow Leopard plus Centos4 & XP Pro in Parallels