To do this you need to work out where the icon is actually going to be displayed. I assume you are putting it into either the QAT, Ribbon or Backstage. In all these cases, you need to edit the ribbonx xml file by using a third party tool many of which are mentioned in this thread
http://answers.microsoft.com/en-us/o...f-8a8aa98e33c6
I use the Office Ribbon Editor from Leaf Creations which you will find gets a mention in that thread.
You also want to find and download the full list of Office Icons so you know the name of every icon. I think this is the link for that
http://www.microsoft.com/en-au/downl....aspx?id=21103