Using the procedure I outlined above, this is how Mozilla Thunderbird handles an embedded graphic:
Date: Mon, 07 Nov 2005 10:28:38 -0500
From: Some Guy <foo@mydomain.com>
MIME-Version: 1.0
To: yabba@dabba.doo
Subject: embedded graphic
Content-Type: multipart/related;
boundary="------------090805080009060203070308"
This is a multi-part message in MIME format.
--------------090805080009060203070308
Content-Type: text/html; charset=ISO-8859-1
Content-Transfer-Encoding: 7bit
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<meta content="text/html;charset=ISO-8859-1" http-equiv="Content-Type">
<title></title>
</head>
<body bgcolor="#ffffff" text="#000000">
<font size="-1"><font face="Verdana">This message has an embedded
graphic here:<br>
<br>
<img alt="dive flag" src="cid:part1.04000608.06020307@mydomain.com"
height="48" width="48"><br>
<br>
This is the end of the message.<br>
</font></font>
</body>
</html>
--------------090805080009060203070308
Content-Type: image/gif;
name="dive_flag.GIF"
Content-Transfer-Encoding: base64
Content-ID: <part1.04000608.06020307@mydomain.com>
Content-Disposition: inline;
filename="dive_flag.GIF"
R0lGODlhMAAwAJH/AP////8AAAAAAAAAACwAAAAAMAAwAEACuJSPqcuNEYCcFMCLs97acFiF
39gJ5BZWZ+atZ0p9rUtf8BQYd81js32z9Ea/UVA4LPKOK+Uww/SZnsuUTkTlXXfZ0y/aLXHA
YSc02DWP0T01iUxyu+DiMJDLmtrPeLk96LcHUrGFI+glAHNIlIi3KBXB9ggxQ7dXZJmmh5H5
pNaptYkiSRUIikhzKiP6QuoSqKEK+SSbw1rlyAH75ponmLk71xucSkpcE3WMC6Pcc9M8ZOUw
TV1tUAAAOw==
--------------090805080009060203070308--