Rush is right for the first part the css is going to control the image being centered on the products page, in my example cart the file is under cart/includes/templates/custom/css/stylesheet_custom.css, with cart being the dir the cart is set to; as a side note your setup might be different and I would suggest using firefox and an addon called firebug which allows you to pick apart pages one section at a time.
The additional images are setup by default to show under the add to cart button though you can play with how many show per row and such in the admin panel. If you would like to change where those images showup, let's say for instance directly under the main product image, you will need to modify the template php file for the product display; which yes that means you need to know php as you'll have to move some code around.
Sorry if the answer is a little long I am sorta detail oriened I guess. Hope that helps answer your question.
Tyler -- iPhoned