You can add pretty much whatever you want it will work much the same way as putting images in the confirmation emails.
I use a bunch of if statements to include images for different carriers/postal services within my order_complete email tpl so that if the tracking number contains a certain string it includes the logo of that shipping company.
Here's an example which is looking for JJD in the tracking number, all my DHL tracking numbers start with this so it's easy to detect.
{if strpos($order.tracking, 'JJD') === 0}
<p />
Your order has been collected by
<p /><img border="0" src="
[insert your domain and/or path to your image folder here]dhl.gif" width="155" height="22">
I have removed my domain name and image path you should substitute your own, but you are just pointing to the image.
I have about 4 different if statements for this so as long as you can identify a unique field and identifier then you should be able to do the same, e.g. if manufacturer equals etc.
Or if you just want the images of a range of manufacturers to appear then simply place ><img border="0" src="
[insert your domain and/or path to your image folder here]dhl.gif" width="155" height="22"> wherever you want the image to show, if you want a bunch of manufacturers images to appear then create a collage of the images and save as a single image and link to that, remember to adjust the height and width to the correct sizes.
I planned but have not got round to it yet to do this for adverts at the bottom of my packing slips as generally there is atleast 2 inches of space at the bottom of them when printed which is a waste of valuable paper
that could be filled with a special offer voucher or simply deal of the week.
The best bet is to copy the template and then play around with it using different things you want to acheive, then when you are happy with the result copy this over the original template (always keep a backup).