From: foo1@bar.net
To: foo2@bar.net
Subject: A simple example
Mime-Version: 1.0
Content-Type: multipart/related; boundary="never-graduate";
         type=Text/HTML
--never-graduate
Content-Type: Text/HTML; charset=US-ASCII

to the other body part, for example through a statement such as:
<img src="cid:foo4*foo1@bar.net">

--never-graduate

Content-ID: <foo4*foo1@bar.net>
Content-Type: IMAGE/GIF
Content-Transfer-Encoding: BASE64

R0lGODlhGAGgAPEAAP/////ZRaCgoAAAACH+PUNvcHlyaWdodCAoQykgMTk5
NSBJRVRGLiBVbmF1dGhvcml6ZWQgZHVwbGljYXRpb24gcHJvaGliaXRlZC4A
etc...

--never-graduate--
# remember to pitch for RFCs We Love