I'm having a problem with Mac Mail.
I'm trying to get a HTML signature to work. I make the HTML and open in Safari, click "File" -> "Mail Contents of this page" and it opens it in mac mail in a new email. Here is what I get:

So far so good. So now I want to cope that and paste it into my email signature. So I select all, copy and paste into the signature:

Here it looks ok, but when I save it (e.g. click onto another signature and back onto this one) the formatting automatically breaks:

Both the "Pharma Mix" bit and the tail of the speech bubble are images.
Why is this happening? And how can I get it to stop doing this?
Many thanks.
Here is the full html:
<html>
<style>
a { color:#fff; }
h1 { font-size:18px; margin-bottom:0px }
h2 { font-size:14px }
h3 { font-size:12px }
</style>
<body>
<table width="435" cellpadding="0" cellspacing="0">
<tr>
<td>
<img src="http://www.pharma-mix.com/header.jpg">
</td>
</tr>
<tr>
<td height="250" style="display:block; padding:20px; color:#fff; font-family: verdana; font-size:10px; background-color:#19346b">
<h1>A Person</h1>
<h2>Job Title</h2>
<h2>00000 000000</h2>
<h3>00000 000000</h3>
<h3><a href="www.pharma-mix.com">www.pharma-mix.com</a></h3>
<p>Address • Here • PO57 C0D3</p>
</td>
</tr>
<tr>
<td height="19"><img src="http://www.pharma-mix.com/emailtail.jpg"></td>
</tr>
</table>
</body>
</html>