Try to insert it directly, this way you can insert multiple images at various locations in the email.
<img src="data:image/jpg;base64,{{base64-data-string here}}" />
And to make this post usefully for others to:
If you don’t have a base64-data string, create one easily at:
http://www.motobit.com/util/base64-decoder-encoder.asp from a image file.
Email source code looks something like this, but i really cant tell you what that boundary thing is for:
To: [email protected]
Subject: ...
Content-Type: multipart/related;
boundary="------------090303020209010600070908"
This is a multi-part message in MIME format.
--------------090303020209010600070908
Content-Type: text/html; charset=ISO-8859-15
Content-Transfer-Encoding: 7bit
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<meta http-equiv="content-type" content="text/html; charset=ISO-8859-15">
</head>
<body bgcolor="#ffffff" text="#000000">
<img src="https://stackoverflow.com/questions/6706891/cid:part1.06090408.01060107" alt="">
</body>
</html>
--------------090303020209010600070908
Content-Type: image/png;
name="moz-screenshot.png"
Content-Transfer-Encoding: base64
Content-ID: <part1.06090408.01060107>
Content-Disposition: inline;
filename="moz-screenshot.png"
[base64 image data here]
--------------090303020209010600070908--
//EDIT: Oh, i just realize if you insert the first code snippet from my post to write an email with thunderbird, thunderbird automatically changes the html code to look pretty much the same as the second code in my post.