[Zope] Altering product instance changes all instances!

Hi For my SiteSummary [1] product, I have a default image that is stored as a subobject in my ZClass. An instance of the product can upload a new image to replace it. Unfortunately that ends up replacing the image in _all_ instances of the product. This is not the desired behavior! Can anyone suggest what I'm doing wrong and how I might go about fixing this problem: ie. creating a distinct instance of the image for every product instance. Thanks Edd. [1] http://www.zope.org/Members/edmundd/SiteSummary