1 product image folder for multiple domains/shops?

Situation:

I've got multiple domains with joomla/hikashop installed on it. The webshops are displaying the same products, but have different banners and ads.
At this moment i'm adding new products on 1 domain and add the product images later to the /upload folder on the other domains. This costs me too much time and it's inefficient use of my serverspace.

I would like too have all my domains to use the same product image folder. Is this possible? and what files do i have to change to get this to work. It would save me alot of time.

Any help would be greatly appreciated.