[SOLVED] How to apply a new css for a "Buy Now" button but not alter current css for

Hello, atahualpa is a great theme and have been grateful and even donated to some who have helped me here.> hopefully this one might be a no-brainer to some of you as it is a real stump to me.

Using Atahualpa 3.5.3, Wordpress 3.0.1

Whenever I put an image on the post it is universally applying a css style sheet. However this is not working when it comes to placing a "buy now" button in your post . By using firebug I found that it is using this style:
.post img {
-moz-border-radius: 3px 3px 3px 3px;
background-color: #F3F3F3;
border: 1px solid #DDDDDD;
padding: 5px;
}
I don't know where this style is located. I thought it was in css.php but it doesn't seem to be there. I don't want to change the current css but I just want a new style to be applied to the "buy now" button so that it doesn't have the same look as the current images.

So how would I go about doing this? Thanks!