To start with, better Web sites usually have more images. For example, they tend to have button bars at the top of each page, to brand the site and to aid in navigation. And instead of displaying Titles Like This in screen text, they often display as gifs. Text rendered as a gif can be antialiased, meaning you don't see jagged edges. When I say that better sites use more images, I do not mean that they use more k of images.
Big images take a long time to download, and that is the kiss of death in an online store.
In a top-quality site, images are the seasoning, not the foundation of the site. Use small, punchy images that will carry a lot of the surrounding area. In particular, avoid the common mistake of putting a huge image on your front page. By all means put your logo on the front page, and in fact on every page, but make it download fast. Your logo is not what your customers came to see.