I have put together a WordPress screencast video on adding a featured thumbnail in WordPress. A thumbnail is usually a small image that represents something like a larger image, or even a small image related to an article.
WordPress is pretty cool to have the ability for featured thumbnails built into the core. However, it is necessary to note that in order to use this feature, it must be built into the WordPress theme being used. If it is not, then this feature will not appear.
This screencast is based on the scenario that the WordPress theme has featured thumbnails built into it.