Macs also understand the ISO format. You can simply double-click an ISO to "mount" it as a mounted volume, or use "Disk Utility" to burn it to a CD or DVD.
There are also IMG, DMG, TOAST, CDR and other disk image formats as well that are "understood" on a Mac. You can use Disk Utility to create DMG and CDR disk image formats, and you can use Roxio Toast to create TOAST disk image formats. IMG is a deprecated image format from the OS 9 days, but Mac OS X can still mount them.
The short answer is that the Mac equivalent of an ISO is an ISO. No changing needed.
If you wish to change an ISO to a DMG, the easiest way is to mount the ISO, then use Disk Utility to make a DMG disk image of the mounted volume.