Eudora:
This is - as it should be - NOT marked as attachment in online gmail Sent mailbox.

<body>
Test message<br><br>
<x-sigsep><p></x-sigsep>
<img src="cid:[email protected]" width=18 height=18 alt="sign">
<font face="Comic Sans MS" size=3 color="#0000FF">&nbsp;<b><i>My_name</i></b></font><font size=3>&nbsp;
<img src="cid:[email protected]" width=18 height=18 alt="sign">
<br>
</font></body>
</html>

--=====================_12345678==.ALT--
--=====================_12345678==.REL
Content-Type: image/gif; name="sign.gif"; x-mac-type="yyyyyyyy"; x-mac-creator="zzzzzzzz"
Content-ID: <[email protected]>
Content-Transfer-Encoding: base64
Content-Disposition: inline; filename="sign.gif"


--=====================_12345678==.REL--

Thunderbird:
This IS marked as attachment in online gmail Sent mailbox.

<html>
  <head>

    <meta http-equiv="content-type" content="text/html; charset=UTF-8">
  </head>
  <body>
    Test message
    <div class="moz-signature">-- <br>
      <meta http-equiv="content-type" content="text/html; charset=UTF-8">
      <title></title>
      <img moz-do-not-send="false"
        src="cid:[email protected]" alt="" width="18"
        height="18"> <font face="Comic Sans MS"><b><font color="#0000ff">My_name</font></b></font>
        <img moz-do-not-send="false"
        src="cid:[email protected]" alt="" width="18" height="18"> </div>
  </body>
</html>
--------------121212121212121212121212
Content-Type: image/gif; name="sign.gif"
Content-Disposition: inline; filename="sign.gif"
Content-Id: <[email protected]>
Content-Transfer-Encoding: base64


--------------121212121212121212121212--
--------------a1b2a1b2a1b2a1b2a1b2a1b2--