While Microsoft Windows 8 setup assistant utility provides and excellent way to download and create operating-system bootable installation media (CD/DVD/USB/ISO), it does not provide any means to use an existing .ISO file for creation of these bootable media options.
To create an bootable CD, DVD or USB setup disk from an previously downloaded and saved Windows 8 ISO file follow the steps demonstrated below.

Download the Windows7 USB/DVD Download tool, and use as shown in the video below to get your bootable Windows 8 DVD or USB disk ready in minutes.