You must have the image stored somewhere where anyone can reach it fast. If not, you will have to build a MIME message with the embedded image which is more complicated and may bloat the message.
We use Imgur to store the images. This is free, fast, and will run "forever".
Here is an example of a file in use as my signature:
<!DOCTYPE html>
<html xmlns="http://www.w3.org/1999/xhtml" lang="da" charset="iso-8859-1">
<head>
<title>E-mailsignatur Cactus Data ApS</title>
</head>
<body style="font-family: calibri, 'segoe ui', helvetica, arial, sans-serif">
<span style="font-size: 11pt">
Venlig hilsen / Best regards / Freundlichen Grüßen<br>
Gustav Brock
</span>
<hr>
<p style="text-align: center; margin-top: 10px; margin-bottom: 0px;">
<img src="https://i.imgur.com/HbdRTBD.gif" alt="Cactus Data ApS" title="Cactus Data ApS" /><br>
<span style="font-size: 9pt;">
Kalkbrænderiløbskaj 4 A · 2100 København Ø · Danmark<br>
CVR/VAT: DK12002696 · Telefon: +45 39 29 49 48 · E-mail: <a title="gustav@cactus.dk" href="mailto:Gustav%20Brock%20%3cgustav@cactus.dk%3e?subject=Re:%20E-mail">Gustav Brock</a>
</span>
</p>
<hr>
</body>
</html>
When done and carefully tested (very important, as Outlook doesn't render exactly as the common browsers), it is a simple matter to create VBA that will recreate it.
Or, alternatively as we do, store the complete HTML in a Note (Long Text) field and retrieve it when creating the e-mail.