Syntax to add image to invoice

I would like to add a logo image that is different from my company logo to my invoices. I have certain products that are certified by various organizations and I want to put the logos of those certifying organizations on the bottom of my invoice. I have put the logos in my \store operations\pictures directory, but I cannot figure out the XML syntax that I need to use in order to make the images show up on the invoice. Any suggestions?

Thanks,

Paul