[Zope] Zclasses

Can anyone give me a simple walkthrough for Zclasses? An initial (and trivial) example could be to create a new object from 'image', called, say, 'picture', with an added long description (textarea) property. Or adding a new property to the User object. Or something. Once I grok ZClasses zen, I'll be happy to write up a How-To or two. Michael Bernstein.