If you use this header in your own articles, you should be aware that only a few other newsreading programs understand and support it. Users are also permitted to override the behavior you have requested. So you shouldn't expect this header to work all of the time. It encodes a small, black and white image, which MT-NewsWatcher displays in article windows if the header is present. Some such headers contain additional information about any transformations that have been performed on the message, or extra information about the recipient.
Some senders also make up humorous or informational headers.
Only do this if you know what you are doing.