.dmg's are the OSX equivalent of .img. Both when opened mount as a disc/volume on the desktop. Disk Copy is the utility that mounts and creates them (sounds risqué).

I have been able to mount some .dmg's when booted in OS 9 by using the OS 9 Disk Copy, but not all.

Basically, avoid the .dmg's unless you're on OS X. Chances are the contents are OS X mostly.

Cheers