Twenty Ten theme how to add image to each post in category listings

I am quite new to this blogging.

But I use the free WP Twenty Ten theme and I want to add an image to each post in the category listing pages.

Any ideas please or point me to a widget.

Many thanks

The page I need help with: https://www.gb-online.co.uk/gb-wordpress/category/sales/

The page I need help with: [log in to see the link]