PEIMG.EXE is no longer located in the WAIK installation directory as the note at the bottom of the article states. I installed the WAIK for Windows 7 (7K3AIK_EN.ISO).
I have read posts that say you should use dism instead (located in WIAK\tools\servicing directory. I have not managed to get it working though.
Would be nice if Microsoft would update the documention....