Rich (HTML) Messages
Rich messages are actually complex documents formatted with HTML (Hypertext Markup Language) in exactly the same way as web pages are. They often have a layout which is based on or includes images, therefore these images are generally meant to be displayed in place. However, since version 2.1 there is a separate option for images in HTML layouts that allows you to display them as icons (except background images and similar special cases).
On the other hand ordinary plain text messages can have images attached to them as well as any other kinds of files. This is where the Iconizer is most useful. It ensures that either no attachments at all or only those you specify are displayed in place (i.e. directly inside the body of the message).
Since version 2.0.2 images in rich messages composed by you are also properly configured to be displayed in place or as icons based on your settings. (These images are automatically withdrawn them from the HTML layout if needed.)
You can configure which attachments are displayed as icons and which in place in the Iconizer's Preferences.
You can also set up Mail not to display remote images in HTML messages (i.e. those which are not embedded in the message and have to be loaded from a remote server). To do so choose Mail > Preferences… and click Viewing. Deselect the “Display remote images in HTML messages” checkbox. This feature is independent of the Iconizer.