Add/modify the following style in your skin stylesheet:

.AddToCartButton
{
display:block;
background-image: url(image/add_to_cart.gif);
font-size: 0px;
width:122px;
height:26px;
color:red;
border:0px;
margin-top:5px;
padding-top:5px;
}

Also remember to either add 'text-index: -5000px;display: block' in your style or set the string resource for that button to ' '