Jingle, when you integrate a pre-built shopping cart onto the site it will come with its own style and images.
Rickster was saying that to make the pre-built shopping cart look a part of your site you will have to change the stylesheet and images that come with it to match. You could leave it as is but it's best to make it look a part of your site otherwise it could discourage sales.
Hope that makes sense
As far as graphic programs go, Photoshop is a great choice.