How to include inline images in email using MailApp

The documention for sendEmail(message) shows how to add two logos to an email as inline images.

Here's an adaptation from Google's example that will prepend your text blocks with their locale flags. It uses images from Wikimedia, which are freely licensed under Creative Commons.

The keys to how this works are:

• HTML <img> tags in the email body use src='cid:[name]' to reference attached images. These are Content-ID Universal Resource Locators, which reference parts of multipart email messages.
• You can use inline style on the <img> tags to tell the mail client how to display them. For example, we've scaled the attached images. (Note: it would be less wasteful to use custom images that are the exact size needed.)

• When the email is encoded for sending, each attached image will be in its own part, labelled with the cid you've provided in the inlineImages object. This object is part of the advanced parameters object for the sendMail() method.

  Here, we have two cids, nlFlag and frFlag, each associated with a file Blob.

  inlineImages:
  {
    nlFlag: nlFlagBlob,
    frFlag: frFlagBlob
  }
  

• File blobs can be obtained in many ways; here we've used UrlFetchApp.fetch() to get the binary images from WikiMedia, then converted them to blobs and set the name of the blob for attaching.

Code:

/**
 * Example of sending an HTML email message with inline images.
 * From: http://stackoverflow.com/a/37688529/1677912
 */
function sendInlineImages() {
  var mailaddress = Session.getActiveUser().getEmail();
  var subject = "test inline images";
  var bodyNL = "This is <B>DUTCH</B> text.";
  var bodyFR = "This is <B>FRENCH</B> text.";

  // Image URLs, under CC license
  var nlFlagUrl = "https://upload.wikimedia.org/wikipedia/commons/thumb/b/b2/Flag_of_the_Netherlands.png/320px-Flag_of_the_Netherlands.png";
  var frFlagUrl = "https://upload.wikimedia.org/wikipedia/en/thumb/c/c3/Flag_of_France.svg/320px-Flag_of_France.svg.png";

  // Fetch images as blobs, set names for attachments
  var nlFlagBlob = UrlFetchApp
                            .fetch(nlFlagUrl)
                            .getBlob()
                            .setName("nlFlagBlob");
  var frFlagBlob = UrlFetchApp
                            .fetch(frFlagUrl)
                            .getBlob()
                            .setName("frFlagBlob");

  // Prepend embeded image tags for email
  bodyNL = "<img src='cid:nlFlag' style='width:24px; height:16px;'/>" + bodyNL;
  bodyFR = "<img src='cid:frFlag' style='width:24px; height:16px;'/>" + bodyFR;

  // Send message with inlineImages object, matching embedded tags.
  MailApp.sendEmail(mailaddress, subject, "",
                    { htmlBody: bodyNL + "<BR/><BR/>" + bodyFR,
                      inlineImages:
                      {
                        nlFlag: nlFlagBlob,
                        frFlag: frFlagBlob
                      }
                    });

}