There's a nice screencast to check out that shows how to use the Ribbon designer and use built in Office icons in your ribbon:

http://channel9.msdn.com/ShowPost.aspx?PostID=345898#345898

Also on the topic of ribbons and icons, check out this great post at Andrew Whitechapel's blog on using transparency when grabbing Office icons for your .NET app:

Preserving the alpha channel when converting images