Product list with parent categories

I have another little question....

My products are arranged in the page "products"
When visitors click on products, i would like them to see all categories with an overview image. i think i can achieve this with adding a product list module with 'parent categories' checked.

However, when i do this i get 'no products found'

How do i add parents to my products? In product management i can only add products.
How can i add parents with images/text which will show up?

Hope i explained it right. I had a look in the manual, but can't really figurere it out....

Thanks!