This PHP code can be placed at the end of your functions file to hook in a image to any custom post type archive.
Simply change the word portfolio in the PHP code to match your CPT name.
You can also change the hook position from genesis_after_header to any other genesis hook.
The CSS needs to be pasted at the end of your child themes style.css file before the Media Queries. It also includes a hover effect which you can also remove or modify.