An in-depth look at how the post image feature in WordPress 2.9 can be used from the perspective of both end users and developers. … You’ll need to call the image somewhere within The Loop in your template files. For example, you might want to add thumbnails to your category archives. You’d do this by adding this line of code: < ?php the_post_image( 'thumbnail' ); ?> Or, maybe you have a section on a special template that calls for medium-sized images: …

The rest is here:
Everything you need to know about WordPress 2.9's post image feature