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.
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Posts may use any of the Perl Monks Approved HTML tags. Currently these include the following:
<code> <a> <b> <big>
<blockquote> <br /> <dd>
<dl> <dt> <em> <font>
<h1> <h2> <h3> <h4>
<h5> <h6> <hr /> <i>
<li> <nbsp> <ol> <p>
<small> <strike> <strong>
<sub> <sup> <table>
<td> <th> <tr> <tt>
Snippets of code should be wrapped in
<code> tags not
<pre> tags. In fact, <pre>
tags should generally be avoided. If they must
be used, extreme care should be
taken to ensure that their contents do not
have long lines (<70 chars), in order to prevent
horizontal scrolling (and possible janitor
Want more info? How to link or
or How to display code and escape characters
are good places to start.