How to change the border width around images?

I am having difficulty changing the border width around the slider in the featured area and the category images on the home page with the titles Hair, Makeup, Fragrance, Skin. I attempted to add a 10px border to the category images and I noticed that now there are borders showing up around other images in posts, lookbook slide shows and other arbitrary images. This is what resulted after I added (border:8px solid black) to .list_category img,#page_holder img{float:left;margin-bottom:15px;border:8px solid black} in the css stylesheet.

My website is: http://www.haute-beauty.com

Thanks