The image should be embedded in the message as an attachment like this:
--boundary
Content-Type: image/png; name="https://stackoverflow.com/questions/9110091/sig.png"
Content-Disposition: inline; filename="https://stackoverflow.com/questions/9110091/sig.png"
Content-Transfer-Encoding: base64
Content-ID: <0123456789>
Content-Location: sig.png
base64 data
--boundary
And, the HTML part would reference the image like this:
<img src="https://stackoverflow.com/questions/9110091/cid:0123456789">
In some clients, src=”https://stackoverflow.com/questions/9110091/sig.png” will work too.
You’d basically have a multipart/mixed, multipart/alternative, multipart/related message where the image attachment is in the related part.
Clients shouldn’t block this image either as it isn’t remote.
Or, here’s a multipart/alternative, multipart/related example as an mbox file (save as windows newline format and put a blank line at the end. And, use no extension or the .mbs extension):
From
From: [email protected]
To: [email protected]
Subject: HTML Messages with Embedded Pic in Signature
MIME-Version: 1.0
Content-Type: multipart/alternative; boundary="alternative_boundary"
This is a message with multiple parts in MIME format.
--alternative_boundary
Content-Type: text/plain; charset="utf-8"
Content-Transfer-Encoding: 8bit
test
--
[Picture of a Christmas Tree]
--alternative_boundary
Content-Type: multipart/related; boundary="related_boundary"
--related_boundary
Content-Type: text/html; charset="utf-8"
Content-Transfer-Encoding: 8bit
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title></title>
</head>
<body>
<p>test</p>
<p class="sig">-- <br><img src="https://stackoverflow.com/questions/9110091/cid:0123456789"></p>
</body>
</html>
--related_boundary
Content-Type: image/png; name="https://stackoverflow.com/questions/9110091/sig.png"
Content-Disposition: inline; filename="https://stackoverflow.com/questions/9110091/sig.png"
Content-Location: sig.png
Content-ID: <0123456789>
Content-Transfer-Encoding: base64
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--related_boundary--
--alternative_boundary--
You can import that into Sylpheed or Thunderbird (with the Import/Export tools extension) or Opera’s built-in mail client. Then, in Opera for example, you can toggle “prefer plain text” to see the difference between the HTML and text version. Anyway, you’ll see the HTML version makes use of the embedded pic in the sig.