I think your proposed solution will work without editing template files in your theme. Below I have created a simple sample template for you to paste in a new template.
System > Transactional Emails > Add New Template
Paste this into Template Styles
body,td { color:#2f2f2f; font:11px/1.35em Verdana, Arial, Helvetica, sans-serif; }
Below is the whole template to paste in to Template Content.
<body style="background:#F6F6F6; font-family:Verdana, Arial, Helvetica, sans-serif; font-size:12px; margin:0; padding:0;">
<div style="background:#F6F6F6; font-family:Verdana, Arial, Helvetica, sans-serif; font-size:12px; margin:0; padding:0;">
<table cellspacing="0" cellpadding="0" border="0" height="100%" width="100%">
<tr>
<td align="center" valign="top" style="padding:20px 0 20px 0">
<!-- [ header starts here] -->
<table bgcolor="FFFFFF" cellspacing="0" cellpadding="10" border="0" width="650" style="border:1px solid #E0E0E0;">
<tr>
<td valign="top">
<a href="{{store url=""}}" style="color:#1E7EC8;"><img src="{{var logo_url}}" alt="{{var logo_alt}}" border="0"/></a>
</td>
</tr>
<!-- [ middle starts here] -->
<tr>
<td valign="top">
<h1 style="font-size:22px; font-weight:normal; line-height:22px; margin:0 0 11px 0;"">Thank you for subscribing to our newsletter.</h1>
<p>Below you will find your $5 coupon to use in store.</p>
<p><img src="[THE ABSOLUTE URL FOR YOUR IMAGE EG: http://www.melissastore.com/media/coupon.png" alt="$5 Off Coupon" title="Print This Email to Claim your Coupon In Store" /></p>
</td>
</tr>
<tr>
<td bgcolor="#EAEAEA" align="center" style="background:#EAEAEA; text-align:center;"><center><p style="font-size:12px; margin:0;">Thank you again, <strong>{{var store.getFrontendName()}}</strong></p></center></td>
</tr>
</table>
</td>
</tr>
</table>
</div>
Note the line that contains
<img src="[THE ABSOLUTE URL FOR YOUR IMAGE EG: http://www.melissastore.com/media/coupon.png" alt="$5 Off Coupon" title="Print This Email to Claim your Coupon In Store" />
You need to substitute the src with your own image's url in the above code. Upload the image using your FTP client.
Best Answer
Try
{{var subscriber.customer_firstname}}
Take a look at
Then take a look at /app/code/core/Mage/Newsletter/Model/Resource/Subscriber/Collection.php