You can install Windows 8 from a USB device in the same manner as you would from an optical disk. You can use the Install Windows 8 button on the page Upgrade to Windows 8 with only a product key to create USB media or an ISO image on a working system. Bear in mind that the media created will be of the architecture of the system on which it is run. For example, if you open the above page and click the button on a Windows XP 32 bit system, the media created will be 32 bit. To create 64 bit media, you will need to ensure the system creating the media is 64 bit.
If you already have an existing ISO image or have made one from a disk as the link provided by SR-71 Blackbird states you can use the Windows 7 USB/DVD Download Tool from the Microsoft Store to create bootable USB media from the ISO file.