[Checkout] customization question

I'm creating a stock photography site and I'm using G2 embedded in Drupal and the checkout download module. On the photo page I would like a radial button and an add to cart button at the bottom. How would I go about that. No one is going to want to buy a large and small version of the same picture.

You can checkout what I'm trying to do is here. Please note I don't like the theme so I'm going to redo it.