Create a gallery with the pictures you want to have in your shop.
In the embeded code, add &galleryname=example+gallery+name after showheader=0. Change galleryname=example+gallery+name to your galleryname.
Examples:
My Venice shopping cart: http://www.colinutzphotography.com/fine-art-prints-shop-by-colin-utz-venice-venedig
My Bangkok shopping cart: http://www.colinutzphotography.com/fine-art-prints-shop-by-colin-utz-bangkok-thailand
Colin Utz
www.colinutzphotography.com