how to boot from windows xp iso image

Are you trying to install Windows XP on your Mac?

From Boot Camp 5.1: Frequently asked questions - Apple Support

Which versions of Windows work with Boot Camp 5.1?

Boot Camp 5.1 supports 64-bit, non-Enterprise versions of Windows 7, Windows 8.0, and Windows 8.1. This version of Boot Camp doesn't support Windows XP, Windows Vista, or any 32-bit version of Windows. Depending on your Mac, you may be able to use an earlier version of Boot Camp to work with other versions of Windows. For details, see the Boot Camp system requirements for Microsoft Windows.

1. What is the version of Bootcamp Assistant on your Mac?

2. What is the model/year of your Mac?

You cannot install XP on this machine using Bootcamp. You will need to use a virtualization engine - VMware Fusion, Parallels or VirtualBox. Choose the version of these tools that supports WXP.

You have an option. OS X: Installing OS X on an external volume - Apple Support and install a version of OSX (Snow Leopard) that supports XP. Use the Bootcamp Assistant from this external disk and install XP and drivers as appropriate.

This method will work with Mavericks but not with Yosemite, because of Yosemite's choice of file systems. It can be done, on a disk with Yosemite, but requires a bit more work.

You will also run into driver issues, because XP may not have drivers for the current hardware.