Currently, there are no built in images for the ribbon and you should supply your own images.
Since the buttons have two images (one large and one small), images need to be either a hybrid 48 by 32 pixels or two images one 16x16 and one 32x32 pixels.
If you need more information about the ribbon you might check the generic topics covered by the help -- http://www.telerik.com/help/aspnet-ajax/ribbonbar-overview.html
-- or if you can't find what you are looking for there, you can write here in the forums / start a support thread.
All the best,
the Telerik team
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