This is a bit out of my league, but I can point you in the right direction:
Look at theme.php:
// HTML template for e-cards
$template_ecard = <<<EOT
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html dir="{LANG_DIR}">
<head>
<title>{TITLE}</title>
<meta http-equiv="content-type" content="text/html; charset={CHARSET}" />
</head>
<body bgcolor="#FFFFFF" text="#0F5475" link="#0F5475" vlink="#0F5475" alink="#0F5475">
<br />
<p align="center"><a href="{VIEW_ECARD_TGT}"><b>{VIEW_ECARD_LNK}</b></a></p>
<table border="0" cellspacing="0" cellpadding="1" align="center">
<tr>
<td bgcolor="#000000">
<table border="0" cellspacing="0" cellpadding="10" bgcolor="#ffffff">
<tr>
<td valign="top">
<img src="{PIC_URL}" border="1" alt="" /><br />
</td>
<td valign="top" width="200" height="250">
<div align="right"><img src="{URL_PREFIX}images/stamp.gif" alt="" border="0" /></div>
<br />
<b><font face="arial" color="#000000" size="4">{GREETINGS}</font></b>
<br />
<br />
<font face="arial" color="#000000" size="2">{MESSAGE}</font>
<br />
<br />
<font face="arial" color="#000000" size="2">{SENDER_NAME}</font>
(<a href="mailto:{SENDER_EMAIL}"><font face="arial" color="#000000" size="2">{SENDER_EMAIL}</font></a>)
</td>
</tr>
</table>
</td>
</tr>
</table>
<p align="center"><a href="{VIEW_MORE_TGT}"><b>{VIEW_MORE_LNK}</b></a></p>
</body>
</html>
EOT;
and look in mailer.inc.php in the ./include dir.
If you know what you are doing with MIME types, etc...you should be able to figure this out.
