Hi,
To be honest I am not sure if I understand what exactly you want to accomplish.
Could you please describe this in more detail?
Do you have a custom document type with two image fields (product and category)?
Do you want to use Product field instead of Category field in transformation when Inherit property is set to true?
Anyway, Kentico
CMS does not support inheritance between document types fields. But there is
Attribute default value property that allows you to predefine value for particular field.
Best regards,
Michal Legen