I was looking through some old notes and found this, hope this helps:

1. PXE boot on the computer you wish to image.
2. At the first prompt, choose the option “Exit to command Prompt.”
3. Map a network drive to your distribution share and run ImageX.  Example:
   1. net use x: \\bddserver\distribution
   2. cd x:\Tools\x86
   3. imagex /capture c: x:\Captures\ImageName.wim “Computer Image Description”

Source:  http://lukenotley.wordpress.com/2007/05/20/bdd-2007-how-to-capture-a-reference-computer-image/