I would try using tables to place the text and images where you want. Perhaps you can take
Code:
{ include file="mail/order_invoice.tpl" order=$orders_data[oi].order customer=$orders_data[oi].customer products=$orders_data[oi].products giftcerts=$orders_data[oi].giftcerts}[img]http://www.painfulpleasures.com/body_jewelry/images/invoice_image.gif[/img]
and replace with this:
Code:
<table border=0 cellpadding=2 cellspacing=0>
<tr valign="top">
<td>
{ include file="mail/order_invoice.tpl" order=$orders_data[oi].order customer=$orders_data[oi].customer products=$orders_data[oi].products giftcerts=$orders_data[oi].giftcerts}
</td>
<td align="right">
[img]http://www.painfulpleasures.com/body_jewelry/images/invoice_image.gif[/img]
</td>
</tr>
</table>
That might work but I can't test it cause my server is having problems
Hope it works