dmg files are OSX specific. You'll need to boot into OSX to open them. Double-click the dmg file & it should open. If not, open Disk Copy & drag & drop the dmg onto it.

When you say image file .img, then either StuffIt Expander should be able to mount it or to use Shrinkwrap (by the same company) or there's others. Those .img files are usually for OS9 but Disk Copy should be able to mount 'some' of them on your desktop in osx. The reason some files are stored as image files is b/c it replicates the CD it originated on. Some softw's can only be installed from the original CD. There is also a utility called Disk Copy for OS9 but it can't open dmg files, only .img files & some others.