Using Windows 8 .img file to create a bootable USB thumb drive
It seems Bootcamp Assistant should be able to help you with this (especially as you have a MacBook Air). It odd that it doesn't. Another weird move from Apple. Because of this you may need to do some system editing, as ethanlee16 points out, to convince BA to give you the option of creating a Bootable USB. 

Follow his instructions, or if you don't want to go into terminal, follow these instructions. 

And then follow these:

First, convert your Windows 8 .img file to an .iso file. You can do this within OSX by following these simple instructions.

Next, just use the new .iso image with Bootcamp Assistant to create a bootable USB drive with Windows 8 on it. These instructions (as posted by Alexandr Mazanov in the comments) do a great job of explaining what to do.

No need for Windows or any VMWare. Good luck!