Image fields
You can use the image fields to enable HTML messages to include embedded images, instead of
      references to images on a web server or the like. Image fields only apply in the HTML body
      text of an email template. These fields are intended to replace the URL against the
        src attribute of an img tag. As with merge fields, they
      are delimited by a combination of {{ and }}, between which are the field type
      (‘IMAGE’), followed by a colon and the image data. The image data must be encoded in a MIME
      Base64 representation, which is a way of encoding binary data so that it is made up only of
      printable characters. As such, the best way of inserting an image in the HTML Body text is to
      select the  option.
The Open dialog is displayed where you can select an image. The supported image types are JPEG, Graphics Interchange Format (GIF), Portable Network Graphics (PNG), and Bitmap (BMP).
After an image file has been selected, its contents are automatically encoded in Base64. The contents are included within an image field at the current insertion point, replacing any selected text in the process.
When a message based on a template with an image field is sent, the image data is
      reconstituted. The image is included as an inline attachment with an automatically generated
      content identifier. Everything between and including the {{ and }} of the image
      field is replaced by a URL pointing to the attachment. For example,
        cid:0e726b30-e45b-11de-8a39-0800200c9a66.