Part 1: Make your picture available online
You cannot directly add images to the mail template, you first need to put them available on the internet. The easiest way to do that is to use the free https://imgur.com tool. (No worries, pictures are private by default. No one without the direct link will see your picture).
Part 2: Add the picture in your mail template
Pictures can be added in mail templates by specifying the code
(no need to add any <%= %> characters).