There are two ways to include images in HTML messages. The first is to reference an image that is served on a webserver. This way works, but requires the user to be online when the message is viewed. The second way actually encodes the image as part of the message so that the user can see the email as intended even if they are not online.
This example demonstrates including an inline image in an HTML message. The first button, "Send with Container", uses the image in the "Container" field. The second button, "Send with Path", uses the path in the "Path" field for its inline image.