[Zope] Altering product instance changes all instances!

[Edd Dumbill]

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