Outlook version?
If 2003 or 2002 you can use the Picture and Mask properties of the
CommandBarButton object and pass IPictureDisp objects, except in cases of
WordMail, where you can't pass an IPictureDisp across process boundaries.
If 2000 or for WordMail you must use PasteFace, which pastes an image that
meets the button image specs that's on the Clipboard. You must put the image
you want on the Clipboard yourself before calling PasteFace.
For 2007 you only want to use the CommandBar interfaces with Explorer
buttons, where you can use Picture and Mask with IPictureDisp images.