Hi @pelican
You would first need to upload them to some web space and the grab the web address (URL) for them.
Then go to Account Settings >> Invoice Customisation and click on the Footer Region. There’s a box there for the PDF footer. Simply add the following for each image:
<img src="URL for file goes here">
For example:
<img src="http://www.my-website.com/image.jpg">
Note: including the http or https is important, or the images won’t load.
I hope that helps, but if you need further help, please don’t hesitate to ask 