Create a bootable USB with Windows installer for Yosemite?

I'm having trouble creating a bootable USB for my mac. I have an iMac, and I'm wanting to install a copy of Windows Server 2008 x86 onto it. I've tried BootCamp, but I'm getting errors when selecting the ISO because it is "not Windows 7 or later".

Does anyone have any idea how I could create a USB that I can boot on my Mac, with my Windows Server 2008 ISO?