Hello Gerard,
If you really want to add your product category image through your cart module, the solution will be edit the code of the "cart" file of the "product" view of your front-end template via "Hikshop->Display->Views".
But the best solution will probably be to set the same image for your products through your product configuration page, and if you have too much product the solution will be to :
- Use the "Export" button of your product listing page.
- Edit the CSV and add the same image for the products which are in your category
- Import the CSV file back.