css classes

I’m trying to create a border on images I place in posts and on pages. By using .img in the Custom CSS, I can add a border to all images on pages, but this also adds a border to the full width image slider and to the captcha image! Does anyone know what CSS classes I need to specify to remove the styling from the captcha image and the full width slider image?

Is there a useful list of classes somewhere: ie, .img, .post_img etc etc

Thanks!
Rx