WordPress finally introduced a Post Thumbnail feature in version 2.9 which has made life quite a bit easier for WordPress theme developers that love to integrate thumbnails into their themes. There are many different ways to dress up post thumbnail images, but for this tutorial we are going focus on a simple 5 minute technique using CSS.

More at: http://woorkup.com/2010/02/15/enhance-your-wordpress-post-thumbnails-in-5-minutes-using-css/