The Visual Studio 2008 Tools for Office team just released a cool suite of productivity apps for VS Office development called the VSTO Power Tools. One of the tools included is the Ribbon IDs Tool Window, an add-in tool window for VS that lets you browse the MSO icons that come with Office. You can use these icons to spiff up your custom Ribbon controls. All you need to do is click an icon in the Ribbon IDs Tool Window to obtain its ID, and then plug the value into the control’s OfficeImageID property.
I’ve recorded a short video that demonstrates the tool window and how to use it to customize some Ribbon controls: