Putting a border on category thumbnail on main page

I would like to put a css black border on my category thumbnail on the main page. Where do I edit the file or the class name?

Another question:
LEFT NAV:
Another problem is the left side margin on the nav and it's alignment and spacing between the pointer arrow and the text menu. Here is the source page that I am copying from:

http://www.wordfortheworldlp.org/adncart/main.shtml

and here is my progress page:

http://www.upload.vemedia.net/adncart/index.php

I'm trying to make it the same.

There is currently only one tag for the image and I do not know if you want this on all images or not. In your stylesheet.css file change to this.

a img {border: 1px solid #000000; }

You seem to have omitted the "better categories" css file but you need to add one or more   spacers for your arrows.