I tried it on an empty .docm file and it worked for me, although the image didn't show up but I am guessing it is a custom icon?
I ran it using Word 2007.
It is strange, since I have been trying to insert a ribbon into Excel all day it has not worked, yet I did a different custom ribbon numerous times last week using the same procedure.
I would suggest finding the most recent Custom UI Editor, dated August 2010. The version number didn't change, but it is definitely newer.
This version is the one dated August 2010, and while some of the comments say it doesn't work for 2007, I believe it does if done correctly.