Using Thunderbird I added an inline image to an email to myself, saved the received email to a file and opened it in a text editor.
The html part of the message looked like this:
<html>
<head>
<meta http-equiv="content-type" content="text/html; charset=UTF-8">
</head>
<body>
<p>Image inline<img src="
cid:part1.yqKN0...@gmail.com"
moz-do-not-send="false"><br>
</p>
</body>
</html>
The "Image inline" is just text I put in the message before adding inline the image. The "moz-do-not-send=false" is just a processing flag for Thunderbird. I sent a second email with the image inline and added as an attachment and this text was no longer there.
Maybe it needs a head and body section to function as a correct html email. Just trying to help by using an email client to manufacture a message of the type you want to send.
Regards,
Ron