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 that is based on or includes images, therefore these images are generally meant to be displayed in place. There is a special option (disabled by default) to alter this behavior when displaying incoming messages in Attachment Tamer Preferences. You can also set up Mail not to display remote images in HTML messages (i.e. those that 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 option.
In addition to images embedded in HTML messages, other files (including image files) can be attached to both the ordinary plain text messages and the rich text messages. Attachment Tamer ensures either none of these files at all or only those you specify are displayed in place (i.e. directly inside the body of the message). Apple Mail without Attachment Tamer usually prefers to display attachments in place, most other programs and web services prefer to display attachments as icons or lists of files. (Also see Content Disposition.)
With Attachment Tamer's compatible format you can safely mix embedded images (displayed in place) and other kinds of attachments in a single message. To send images embedded in HTML the message needs to be in the rich text format.
How to set message format
You can use rich text formatting in an individual message by choosing Format > Make Rich Text when composing the message, or you can set rich text as your default message format in the preferences: choose Mail > Preferences… and go to the Composing pane. You can choose the plain text format in the same way. Attachment Tamer ensures that the format you select is always respected (otherwise Mail would sometimes sent plain text messages unless you used at least two different text styles in the message). If you are composing a message with images that are to be displayed in place, using rich formatting (and so embedding the images in an HTML layout) is the best way to ensure the message will be displayed consistently across different software platforms.