Adding image to category archives

Hello, I am trying to figure out how to change the icon in “categories” to an image I have but cannot figure out how. I can’t seem to find anywhere to do this. Every time I create a new category the program ads a new icon and would like to replace the sample icon.

Here is the link to the page. http://www.photographydesign.org/Temp2/category/actors/

Thanks

Ed