Want to add a small icon in the corner of a product Picture

Question

I want to be able to add a small picture into the corner of products that are within a certain category. For example I have a Featured products category which is used on my home page to display certain products. However i want to add a small image to the corner of these products so customers know they are Featured products even when the find the products in there own categories.

If i havnt explained myself very well please just let me know and i will try explain my problem better.

Thank you :)