[Zope] Renderable Base Zclass

Hi, I'm desperately trying to understand how to make the CaptionedImage as an example to use Renderable Base ZClass. I just can't get it to work, because I don't really know how to start (me-is-really-new-to-Zope). So I add a product MyImage and add a ZClass to it. The ZClass subclasses Image and Renderable. I got that far. Could somebody explain how to continue? What i basically want is an ordinary Image (with the normal properties) + one extra: the caption, that gets rendered just below the image. I read the ZClass Guide, but the examples in there just don't apply to this one (it seems). Thanks, Jeroen