/skin1/mail/html/order_invoice.tpl
<img src="{$ImagesDir}/companyname_small.gif" alt="" />
In the future, you could find this out yourself if you simply use "webmaster mode" in the admin. Find the template you need, look at what's going on, make your changes...
Without webmnaster mode, there's no way a newbie can know where every item lives... but with...