Okay I feel like an idiot because this took me weeks to figure out, and the solution was so simple - staring me right in the face. But I scoured the forums and couldn't find an answer, so I am posting it here.
To get rid of that annoying black border around your buttons, simply add style="border:0" to the end of this line of code. Once I figured this out, I was able to match the buttons on the wishlist page as well. Now everything looks seamless.
THX for your speedy reply; I should have known but did not entirely know where to put the line.
Because I have several images per product which reflect actually the attributes of a product I name the images, eg duchess-blue.jpg, so for the benefit of customers I changed the line in product_info.php a little bit to show the name of the image as well as the name of the product.