SMTP, the standard email protocol, does not specify support for anything like a "favicon". While there may be some email clients which add it as an extension, they are exceptions to the rule. Even if you were to arrange for your image to appear in the subject line on one mail client, the vast majority will not display it at all (indeed, many aren't even capable of displaying an image - text-only mail readers are still around) and, even among those which display it, most would probably display it elsewhere.