Currently the cart category module does not support images but there is a solution that will allow you to accomplish your requirement. Using dnn create your own separate category page and use the html module to create category links and images that link to the page with the buynow module.
The product listing page of the cart allows you to filter products by category by passing the category id in the querystring. For example:
http://www.airsoftguns360.com/Home/tabid/38/Default.aspx?categoryid=1
When the category id is passed in the querystring the products are filtered by category.